Why does Finny need Gene?

I can't recall the exact statement but I do know that by the latter part of the novel their relationship becomes almost symbiotic. Finny needs Gene to realise his goal of going to the Olympics. If he can't go, at least he can train Gene to. There is much more to this though. Finny's accident meant a major loss of identity. Finny's whole world was that school and his social purpose around it. Gene was a way, in his mind, to keep that consistency. It was a way to prove something that he felt he no longer could as a "cripple". Gene also needed Finny but in a different way. The relationship, if you have not yet noticed, was not entirely healthy.