Character Analysis
The great thing about real people is that they don't need a lot of embellishment to be rich or fleshed out. Life does all the work and the author just reaps the rewards! In this case, the author knows his hero pretty well, since it's just a younger version of him. And we get a front-row seat to both his good qualities and his bad.
Liar, Liar
Jack's manipulative; we learn that early on when he "makes a play for souvenirs" (1.4) he soon graduates into full-blown lying. In fact, he's so good at it that he sometimes convinces himself that his lies are the truth:
It was truth known only to me, but I believed in it more than I believed in the facts arrayed against it. (22.76)
That kind of wishful thinking isn't usually good for your health… though it actually kind of feeds back on himself, since he can argue in favor of his made-up stories much more strongly than he would if he knew it was a lie.
Sounds like a recipe for a nogoodnik, but there's a method to his practiced deceit. He doesn't especially like himself:
I was subject to fits of feeling unworthy, somehow deeply at fault. (2.13)
Notice that there's no root cause here: he's just "somehow" at fault.
So if he's making up his faults, he can make up a phony façade to plaster them over. He wants his lies to make him into a better person:
I could introduce myself as a scholar-athlete… and without any reason to doubt me, people would believe I was that boy, and thus allow me to be that boy. (10.11)
At least then they serve some kind of purpose in his life. They're an expression of his need to belong, to be liked by others and to be the sort of person that he would like to be. It's destructive because they don't work… at least here. But we can infer that they eventually give him a kind of redemption, since he learns how to channel them creative through writing:
And on the boy who lived in their letters, the splendid phantom who carried all my hopes, it seemed to me I saw, at last, my own face. (22.105)
So there seems to be an upside to it… eventually, once he gets turned around. Which isn't what we see here: we just have to assume it from the fact that he's actually writing the book (and thus turned his creativity to less destructive ends).
Orderly Criminal
Along with his lying comes a healthy dose of hooliganism:
I was a thief. By my own estimation, a master thief. (7.18)
Here's where his need to lie catches up with him; he's not a master thief, but if the thinks he is, he's going to take risks that may not pay off. He also drinks, smokes, stays out late, and steals gasoline from the occasional farm. And he doesn't feel particularly bad about it:
In the past, I had been ready, even when innocent, to believe any evil thing of myself. Now that I had grounds for guilt, I could no longer feel it. (15.11)
Clearly, the irony isn't lost on grown-up Jack, who can observe it objectively without emotion.
His lack of guilt is ironic because he has a specific need for rules and order. He's attracted to soldier's uniforms, the Boy Scouts, anything which promises a sense of structure:
I liked all these numbers and lists because they offered the clear possibility of mastery. (11.28)
He needs to feel like things make sense. Unfortunately, they don't—at least on this plane of reality—and so he rebels. Foolishly. Shortsightedly. Often illegally. But what are ya gonna do? The rules just keep letting him down. The push and the pull of that forms the big struggle of the book… besides, you know, avoiding Dwight's thundering alcoholic rages.
Good Son?
For all of Jack's wild ways, he really loves his mother and wants to make her proud:
She needed me and to be needed made me feel capable. (6.73)
Codependent, yes, but also caring. In his mind, if he can make her safe, he'll be a good person. And just in case we miss the point, he flat-out tells us "I was my mother's son. I could not be anybody else's." (16.37) Furthermore while he "being adopted by different people I saw on the street," (2.16) he eventually rejects a real offer of adoption because it will take him away from her. Win or lose (and good Lord, but there's a lot of losing), he stands by his mother in all things. That helps give him some sympathy in the midst of all the vandalism and general swinish behavior. It might even help explain why he sticks around despite Dwight whaling on him all the time. As much as a he fails at it, he honestly wants to be a good son.
In the end, we probably root for Jack because he's an underdog. He's been dealt a bum hand and he doesn't always play it well, but he plays it as best he can. His failures might feel a little like ours sometimes, and his desire to do better means that we want him to pick himself up whenever he gets knocked in the dirt. That's enough to put us firmly on his side, no matter what kind of 50s punk trouble he might get into.
Jack's Timeline