This is basically summarizing my thoughts on why Jim does this and that and it outlines his life up to the Enterprise (and a bit on Delta Vega too). I’ve also added some stuff about the Reboot!Verse and how it’s different from the TOS!Verse. Also, this essay might be retconned at a moment's notice WHAT WILL THE NEW MOVIE BRIIIIIIING. I might add new stuff as people bring me more info, insights etc!! WHICH I TOTALLY WELCOME /o\ ;;
Early Years
KirkPrime had a fairly simple childhood. He lived on a farm, he could cook, he had a horse and his only fear was a particular fence he couldn’t ride over. That was it! Very idyllic, perfectly simple life. (lalalala Tarsus IV lalala)
Reboot!Jim didn’t have that luxury. I’m counting the deleted scene and parts of
his entry at Memory-Alpha as canon since it does a better job of explaining Jim’s motivations (and I think it is a good way of showing he wasn’t all that different to KirkPrime in the beginning). I've linked the scene
here and adding my own interpretations on it. Basically, Jim's life consisted mostly of George (Junior, aka Sam) and Frank, Winona's brother. To Jim, his parents were a pretty distant concept at that age. His mother resumed work at Starfleet (I think for KirkPrime, his parents retired soon after his birth) and probably came home every six-nine months (shore leave tended to take place over very long periods of time. Often, they were irregularly scheduled so the length of time might be longer) which means Jim's two constant figures and who his life revolved around were Frank and George, who butted heads all the time. Frank was an alcoholic and an abuser, so while George had no problem fighting with him, Jim was trying to keep the peace in the house, often doing what Frank wanted without a moment's hesitation and making sure Frank had no reason to get angry with him, often covering for George as well and doing George's share of chores and whatnot. Ironically, this ended up with George being resentful towards Jim as well. Things escalated and George runs away from home (I think it was his first real attempt. I would imagine that George often threatened Frank that he would leave if he didn't stop mistreating them). It was easily the worst day of tinyJim's life. One, Frank rendered most of Jim's action for his life meaningless ("Nobody cares what you want. You're no one") and two, George was upset at Jim and his main complaint? That he doesn't know what it means to be a Kirk (of course, this was aimed at himself, but it affected Jim as well).
So Jim went back to Frank, upset and frustrated, with no idea what to do next when the car keys fall into his lap. Then every inhibition Jim ever had just vanished. While KirkPrime had some understanding of fear, Reboot!Jim lost it completely. He just gave up on caring.
The PunkTeen Years (and Captain Pike)
I think this was the most dull cornerstone of his life. For KirkPrime, Sam left home fairly early on, got married, got a life and things were decent between them. For Jim, George probably left the minute he hit eighteen (I don't know where or what he did thereafter. For all I know, he enlisted in Starfleet! Here's hoping he didn't go to Denova...) while Jim was still grounded in Iowa. His mom must have retired at some point and Frank vanished (or went to jail??? THROW ME A BONE HERE CANON) which left Jim completely aimless. The whole rebel thing stayed because he had no idea what to do with himself. He lacked motivation to try anything, he lacked faith in people (and himself) and the only thing that kept him going was finding a challenge, any way he could. I imagine he took those Starfleet aptitude tests simply because they were there and they have him something to do. But despite making scenes and juviehall etc, he never sought to make himself look good. He outrightly refused to, despite the fact that would have been easy for him to accomplish. (It might be partly because the past made him sick of any kind of good image he could have cultivated for himself. It could be due to his father's reputation which he wanted to avoid, the ideal of being a Kirk. Hard to say)
Then Captain Pike stepped in. It's interesting to note that Jim's body language throughout their conversation as he makes a habit of trying very hard to ignore Captain Pike. When Pike addressed him as his father's son, he called for another drink. He deliberately tries to be rude and uninteresting (I think the interest Captain Pike invests bothered Jim. It didn't make sense to Jim why he would even bother until Pike told him to enlist) . The line that really hooked Jim was this one.
"Your father was captain of a starship for twelve minutes. He saved eight-hundred lives, including yours and your mother's. I dare you to do better."
And well... Jim can't resist challenges. He can't. Pike gave him a direction; be captain and Jim, once he took a good long look at the docked Enterprise, realized he didn't have to pander to his past. That was the second time he underwent a radical change of thought and broke free from his cyclical life in Iowa and he upped the stakes to Pike.
"Four years? I'll do it in three."
It wasn't a boast so much as it was Jim setting the bar for himself. For the first time in his life, he was truly in control.
Academy Years
Hahaha, other than the Kobayashi Maru and Gaila, Bones and Uhura, we have no idea what Jim's academy life was like. The official website lists him as lieutenant, being the treasurer for the xenolinguistics club, assistant in the advanced hand-to-hand training and top of his class in tactical analysis and survival strategies (though I don't think he played chess. SORRY GUYS). While KirkPrime was fond of classical literature, Reboot!Jim was more of a history buff, familiarizing himself with cultures, their context and so on. Also the Reboot!Verse has strong similarities (slang, words, minor details) modelled after the 21st century so I play Jim with a 70/30 knowledge of things in the past. KirkPrime had some interesting Academy relationships too (Finnegan, Ruth... his homoerotic thing with Gary Mitchell) but I don't think Jim had a very eventful academy life beyond sleeping with half the school and hacking the Kobayashi Maru with Gaila's help.
Enterprise and Delta Vega
The problem with Jim is he really doesn't think. Well, he does, but when he gets angry and emotional, he really really doesn't. While Spock himself wasn't in the greatest mindset after the destruction of Vulcan (because marooning people on icy planets is totally okay), Jim didn't exactly handle it brilliantly himself. He was in the right and he knew it. What frustrated him was Spock's inability to see that as well, and when Jim gets frustrated, he just... takes it out on everyone.
The cave scene with SpockPrime a turning point for Jim because his attitude switches from lashing out indiscriminately to channeling his abilities with a plan. I think what surprised him and what he began to believe in... was himself. SpockPrime pretty much confessed everything to Jim without a thought and then told Jim he had to gain control of the ship. He had unconditional faith in Jim Kirk, which really solidified Jim's desire to back on the Enterprise and turn things around.
Political Atmosphere (aka THE MINOR DETAILS)
This doesn't directly affect Kirk, but they are notes I want to keep track of, since Reboot!Verse =/= TOS!Verse. Oh, and I'm ignoring everything else Trek-wise.
-According to the novel, George Samuel Kirk was able to identify Ayel as Romulan due to his likeness to the Vulcans. Which means at that period of time, the Romulans were still keeping to themselves and the rules of the neutral zone were still active. However, with the destruction of the Kelvin and the disappearance of the Narada, the treaty was most likely weakened. The Federation became a "peace-keeping armada" and the fact that Uhura could speak the Romulan language at all means there were concessions from the Romulan side. [EDIT - There is a neutral zone between the Klingons and the Federation. SAME ONE OR DIFFERENT ONE? HARD TO SAY]
-Also, the Klingons plotting at Rura Penthe means the Klingons don't have an alliance/treaty with the Federation and that relationship hadn't been established yet.
-I've stuck with reboot!verse also needing money like TOS, mostly since there's no mention of it in the movie. Not to say that they have no idea of it, just that it's redundant in reboot!verse as well.
-Orion girls! Firstly, in TOS, Orion girls were mostly slaves/prostitutes and a lot of them were owned by Starfleet captains. They were also supposed to be irresistible to all men. Obviously not the case here. Gaila is an actual student in the Academy (along with another Orion). However, seeing them at all is rare (hence Jim's... mistake). Whether the Federation has a strict policy of people with Orion or the planet keeps itself in isolation for its own safety, I DO NOT KNOW).