Earlier on, the author does speculate about what drove Jack:
> To put it another way, Jack was painfully aware that his future options were, “be a complete loser,” or “be a complete loser who feels really really good for a few hours every day.” He chose the latter.
What's striking about this is how it's possible to live this way without drugs. A brain-numbing job eight hours a day and a life-saving hobby for four. A toxic-family life but wonderful community.
It almost sounds like Tennis could have been this outlet:
> One time when Jack was in middle school, he walked off the tennis court after a well-played match, and his mother asked him how he felt. Jack said something like, “when I’m out there, it’s so nice… it’s like the rest of the world goes away and I don’t have any problems.”