In A Separate Peace, Gene chooses to be an assistant crew manager for several reasons.
1. Gene is feeling sorry for himself and looks for a role that demonstrates his feeling. This is his senior year, and he is at it without his friend Finny. He feels great guilt for what he has done to his friend, and the truth has not been revealed. This role is for an underclassmen, but when given to a senior, it is usually a senior that is less than.
2. Finny encourages Gene to get involved in sports. This is a sport through which Gene can be involved without doing well and showing Finny up. He doesn't want to beat Finny at what Finny is good at.
3. All kids had to be involved in a sport. It was a school requirement.