The only son of two alcoholic parents, Jon's life was never an easy one. While his parents were never abusive the almost constant alcoholic daze they lived in cause him to be self sufficient at an early age. It also meant he missed out on the things other kids take for granted like birthday parties or acknowledgement from his parents on just about anything. This naturally led to him resenting his parents and lashing out in various ways to get their attention.
The most obvious way was to cause as much trouble as he could in school to see if they would ever show up the multiple times the principal would call to report his bad behavior. They never did. This only made him act out even more, causing problems for his teachers and local authorities yet only enough he never seemed to pick on any of the other students. In fact he had been seen multiple times stopping his delinquent friends from harassing the weaker and younger students.
It was this redeeming factor that caused him to be chosen by the Guardian to be pulled into the Digital World and become part of the DigiDestined. Naturally he had a bit of a problem fitting in with the other kids at first and lashed out at them verbally quite a few times, especially rich girl CiCi, but over time he began to warm up to them, even if he wasn't going to outright admit it.
It didn't hurt that his Digimon partner, Quillmon, was about as anti-social as him but in a much more conniving way. In fact "Con man" is the best way to described the prickly Digimon as he sees every other Mon, and now human, as a mark in his crazy plans to get rich quick or just have a laugh at someone's expense. What effect Jon's strange moral center might have on money happy 'Mon is yet to be seen.