Inspired somewhat by
playingmyrole's
meta on Shawn's likeability. I went off topic in the comments responding to it, and subsequently felt like giving that off-topicness its own post. References through all aired episodes.
Here is what we know about Shawn as a kid:
Shawn was drilled almost relentlessly by his cop father. Shawn was not happy about this. (Pilot, and pretty much every episode after.)
Shawn was bullied. (Who Ya Gonna Call?)
Shawn was evidently a fairly good student, although at least part of that can be chalked up to the observational/deductive skills Henry drilled into him: he memorizes the teacher's answer pattern rather than actually learning the material in Who Ya Gonna Call?, for instance.
Shawn was not allowed to read comics. (Shawn vs. The Red Phantom)
Even playing hide and seek or building a model rocket got turned into a competition or training, thanks again to cop father. (Woman Seeking Dead Husband, Weekend Warriors)
Shawn was very bad at finishing projects that he started. (Spellingg Bee)
Shawn's dad never told him he loved him. (Weekend Warriors)
To be honest, I'm consistently shocked that in the flashbacks we keep getting shown a fairly well-adjusted kid who is usually surprised by his dad's expectations and tests. Mom must have been an incredible woman; otherwise, Shawn should be a whole lot more screwed up than he is.
Conversely it is possible that Henry was usually a good dad who actually invested in making sure his kid was well adjusted instead of just a good cop, and it's chance that we have only seen him being the stern taskmaster in flashbacks. But I don't find this terribly likely.
Here is what we know about Gus as a kid:
He almost won the spelling bee. This was important to him. (Spellingg Bee)
He got some of the spillover from Henry's training of Shawn by virtue of being in the same area. (Weekend Warriors)
I really can't tell what Henry thought of Little!Gus. There's that flashback at the beginning of Red Phantom where Henry just sounds so completely disapproving of Gus-who-reads-comics-and-corrupts-Shawn-into-reading-comics, and then there's the Weekend Warriors flashbacks where Shawn tells Gus flat out that Henry respects Gus more than he respects Shawn. It's confusing.
In the present, we have cool-kid Shawn who refuses to grow up and who drags his reluctant, geeky sidekick Gus along with him. You'd think this was a natural outgrowth of their relationship as kids. And reading some of the flashbacks supports this reading, notably Spellingg Bee.
But Weekend Warriors upsets that whole reading, for the scene in which young Shawn wistfully tells young Gus that "Someday I'm going to be as cool as you." And Who Ya Gonna Call?, wherein Shawn is bullied and has to talk his way out of it.
I'm halfway tempted to write it off as inconsistency on the writers' parts, but there's no fun in that. Instead:
We have a young Shawn, who is drilled endlessly by his father. He resents this, but can't actively rebel. Mom is still at home, and she helps him cope. Shawn obediently learns everything his dad drills into him, does fairly well in school, and resents it every step of the way. Hence the opening scene of the pilot, in which he provides Henry with the information asked of him but does it grudgingly, whining and complaining every step of the way.
Now young Shawn meets young Gus. Gus is smart and fun and interesting, and importantly, his family is sane. Shawn admires Gus. Shawn is smart and charming and funny, and also has some scary talents. Gus is entertained by Shawn. They become friends.
Neither of them is cool as far as the rest of kiddom sees it, because they are weird and geeky. But Gus is better adjusted than Shawn is, and so Shawn admires him and considers him cool within their own circle of two. I don't really see either of them having close friends outside of each other.
Now, the two of them grow up together. They go through high school together. There are girls and things. They have fun.
After high school, Shawn, determined not to grow up and rebelling against his father, drifts aimlessly. He has 57 jobs (52? I don't remember and am too lazy to check) and can't hold any of them. There are possible signs of ADD, or else just a really short attention span. Gus, in contrast, applies himself, becomes responsible, holds down a good job, and becomes increasingly stressed, probably in large part because Shawn is still his best friend, and Shawn keeps dragging him into insane situations. (Like at the border of Mexico. Twice.) And so the situations are reversed: Shawn is the "cool" kid, in his estimation anyway; Gus is much more tense and uptight.
It's sad to note that they still don't really seem to have friends outside of each other. Gus notes it explicitly in 9 Lives: "You are my only non-work friend." Which makes you feel terrible for him, until you note the fact that as far as we've seen, Shawn has no friends outside of Gus. He's on friendly terms with lots of people because he is outgoing and charming, but that never goes beyond surface small talk; he has no work friends because prior to the pilot he has no steady work. There's a possibility that Juliet O'Hara could eventually make it to "friend" status with both of them, but she's not there yet, and given that Lassiter actively hates Shawn it's going to be a lot longer before he gets to that point.
Shawn isn't half as well-adjusted as he pretends to be, or as he comes across in a casual viewing. And it's really a miracle that he's made it through life this far, and it's probably due in large part to Gus's influence on him. Hopefully working on serious cases (murder, pretty consistently) with Psych and the SBPD will help ground him some more.