In my 30+ years in and around high school athletics, I have found two reasons why seniors quit.
1. Practices and games are interfering with their "social activites". Boyfriends, girlfriends, partying etc. They are "in love" or so they think.
2. Younger, better players are coming in and taking their playing time. It is much easier to quit than play behind a freshman or sophmore. Putting the blame on the coach soothes their fragile egos.
Having competed against Goodpasture for many years, I would say the reason is a combination of the two. If the player really wants to play they will no matter what. I despised my high school baseball coach, but I loved the game and stayed out there. That is what true athletes do. Also, Trojanman, how do you expect to get better if you don't work hard. I hope you don't coach making a statement like that. Perhaps the lack of work ethic got them to this point in the first place.