Unofficial Timeline (for reference) A map of Dragon World. Personality
AKA: Mun is big geek.
One of the beautiful things about Dragon Ball Z is that there is a lot of room for character speculation. This can be taken two ways: either that the little hints provided through the series give you a lot to work with, or that someone is too lazy to give certain characters a lot of depth. I've created unholy amounts of headcanon concerning Goten, which I will put here. If by some chance you actually read this entire thing, I hope it explains why Goten acts like he does, occasionally; if there's anything you disagree with/would like to comment on, then PLEASE do! Anon commenting is permitted, IP logging is turned off, please feel free to express yourself without worry!
Anyways, moving on.
The first thing that people note when referring to Teen Goten is the fact that he doesn't train; this is usually viewed negatively, because first of all he has astounding potential to become ridiculously powerful, if only he'd keep to the program. Secondly, because he was so avidly into fighting and learning about martial arts when he was seven, so why would he suddenly drop something he loved so much? I attribute this to several factors.
-Goten is a teenager now. While it's a little stereotypical (but c'mon, it's DBZ), I'd think that once Goten hits "that age," he's probably going to be more interested in girls and hanging out with his buddies than mucking around with his dad in the back yard and spending hours upon hours getting smacked around by family and friends. He's moved on to a part in his life where T&A hold a lot more power over his thought processes than when he can spar with Gohan again, and having lived a good portion of his life up in Nowhere on top of some secluded mountain, is it really that hard to believe that he'd want a bit of a social life?
-Fighting for your life at seven can be a tad scarring. Gohan had to do it, too; battle after battle, from when he was as tiny as five. Is it really that much of a surprise that he craved a normal life, also? The same goes for Goten. It's one thing to spar with your best friend in the back yard or for your mom to show you how to stretch properly; it's another thing to watch your family killed by a monster and see your best friend get floored in a life-and-death battle. It's intense and it's scary, and I think after time Goten began to dislike that part of his life, as he reflected on it.
-Being a half-Saiyan freak is not good for the popularity polls. Think about it; sure, there are people that can make good money off of fighting, but if someone were to find out that Goten was a half-breed alien weirdo, he'd be regarded as a freak of nature. Considering how shy Goten can be, it's really not much of a surprise that he'd want to keep a low profile about that kind of thing. Plus, don't aliens usually get grabbed and experimented on? No thank you. Not to mention, if someone finds him out, then Gohan, his dad, his niece and maybe even Trunks get found out, too. Everyone's lives could be ruined by a little slip up, so he's got to keep it as quiet as possible.
-It's part of his nature, and yet it isn't. Obviously, Saiyans love fighting. It's their lives, when they breathe, when they speak; battle is everything, even if it's just sparring. And all of the Saiyans, demis and quarters included seem to really enjoy fighting once they get into it. True, only the full-blooded Saiyans Goku and Vegeta seem to like those bone-breaking, blood-flying, rip-roaring battles, but even Gohan (who supposedly is the mildest of the bunch) enjoys a good fight now and then. However, what about the other half, his human side? Confrontation is naturally something a normal human avoids. So it's his nature to crave battle, but also avoid it? It has to be confusing for him at times, and I'm sure that occasionally it can even be at least a bit of a problem.
Another huge factor in Goten's personality development is his family, and Trunks. They've had a huge impact on shaping the person he grows up to be, and each in their own individual way have both encouraged and discouraged him.
Chichi
Mother. She's always looked out for Goten; it's very obvious that she loves him to and beyond the stars, and would do anything for him. She can be a very strict mother, and has probably more than once given him a spanking when he was young for being disobedient, but in the end it's worked well for him. He seems to be a very polite, well-adjusted kid, so she did her job well. She's not nearly as strict on him as she is with Gohan, and by the time Goten reaches his teenage years it's a little obvious that he's somewhat spoiled in comparison. Maybe not in terms of material items, but he's gotten away with a hell of a lot more than Gohan. He's probably also developed a bit of a mischevious side to help him slide out of trouble, as the baby of the family usually does. Despite all this, Goten adores his mother to his very core, and will defend her against slander and physical threats to the very end of his life.
Goku
Father. This is a complicated relationship for Goten. He grew up on stories of Goku from Gohan, about how he was noble and fair and always gave everyone a chance. That Goku loved his family dearly, and that he was a savior of the world several times over, the strongest man in the universe. Because of this, Goten holds a remarkable amount of respect for Goku, but this is counter-productive in that it also serves to separate them partially. Goku wasn't around for Goten's earlier years, and then suddenly shows up as the legend he is and saves the world again. It can be a little intimidating to have someone like that for a father, and Goten occasionally lets his insecurities take over, thinking he may not be good enough to be the son of this incredible man. Goten also has slight abandonment issues; while he doesn't know that Goku refused to return after the defeat of Cell (he just knows that he didn't come back to life), Goten feels as if maybe Goku had known about him, he'd have tried harder to live. Of course, that's a horribly insensitive thing to say, so he'll never say it aloud.
To make matters worse, Goku left to train Uub roughly a year ago and hasn't so much as called or dropped by for a hello. The fact that Goku ran off to train someone else when he had his own son and his own family to look after really irritates Goten; he has a very firm set of beliefs as to what a father should be, after all. A father should be strong, good, kind, should stay and protect his family and should never hurt his children. But Goku never really sticks around; Goten did some information digging after Goku left and found out that Goku stayed on the planet Yardrat for an entire year after Frieza's defeat because he wanted to train. He also found out that Goku had the opportunity to return to life after the Cell Games and refused, partially to protect the world but also so he could train in the afterlife. So Goten's a bit bitter about Goku right now, and would probably be very touchy on the subject.
Gohan
Older brother. Gohan is Goten's hero in the flesh. Gohan is strong, nice, smart and always played with Goten, no matter how busy he was. If Goku was the legend in Goten's life, then Gohan is the successor. Goten truly believes that there is nothing Gohan can't do, and believes that if he ever gets into trouble that Gohan will come to save him. There is a little bit of insecurity because of how amazing Gohan is; when your older brother succeeds in almost everything he puts his mind to, how can Goten compete? But he could never, ever hold that against Gohan, no matter what. Gohan will always, always be Goten's own personal Jesus, and the one person who will always come through for him.
Trunks
Best friend. If there is one person outside of his family that Goten would do anything for, it's Trunks. Trunks was his only friend when he was smaller, and was right beside Goten through the entire battle against Buu. Goten feels safest with Trunks at his back, and knows that in return Trunks feels safe with Goten watching his. While his friend did usually get them into trouble, he was also usually the one to get them right back out. Out of everyone in Goten's life, Trunks is the one that understands him the best, and Goten feels safe telling Trunks anything and everything regarding his fears, dreams, anything personal to him. Ever since fusing, also, Goten feels like he and Trunks are closer in soul and spirit, and nothing can perk him up like seeing his best friend dropping out of the sky to say hello. There is some admiration and maybe even a bit of hero worship involving Trunks; the boy is smart and strong, and always seems to keep a cool head in times of crisis. If Trunks is rattled, Goten is more than likely scared as well, but when Trunks keeps his wits about him then Goten finds it a lot easier to calm down. Trunks directly affects Goten's moods, and as such can motivate him to do almost anything.