A Separate Piece Quote Significance Chpt 4

Quote
"I found it. I found a single sustaining thought. The thought was, You and Phineas are even already. You are even in enmity. You are both driving coldly driving ahead for yourselves alone."(53)
"Now I knew that there never was and never could have been any rivalry between us. I was not of the same quality as he."

Significance
Thinking is a dangerous game, and like any other sport you have to be aware of and accept the possiblities. In football, you have to risk getting your knee taken out by the 250 pound defensemen as you try to run in the touchdown. In baseball, you run the risk of being smashed in the face by that leather sphere wizzing past your head at 70 miles per hour. In thinking, you risk realize, true or not, something you never wanted to think about. You go out of your comfort to zone to see what is real, and what is not. Sometimes you do not like what you find, and that is the risk you take, but to me, it seems like more than any injury a sport can give you. In football, your knee will heal over time. In baseball, the swelling will cease and the bruises will fade. But with thinking, when you realise something, it is with you forever. No matter whether you ever want to remember it or not. I think that is the hardest lesson Gene learns in this chapter. But his mistake is he gives into paranoia, and makes assumptions about Finny's motivations, about his true goals. His emotions change his actions, and he becomes angry at Finny. Finny did nothing, and want to try to apologise for whatever it is that Gene is blaming him for, but Gene childishly refuses such and makes a rash decision. Gene is over thinking things way too much about everything and cannot just take things as pure or natural. This really will come back to haunt him later, I know this.

Question
Why does Gene feel this way? Or why does he over think things? Was he just born that way or did something happen?