His decision of wanting to leave can be due to the way he's being treated at the club to the salary and wages that he's earning and wants to leave for a greener pasture and also could be a matter of the individual player wanting change of environment, Therefore, If a player decides to leave a particular club for another, the best thing the management should do is that he should be given that opportunity because refusing him will not really do the team any good as he won't be playing with he's total mind and strength and offering his best to the club because he had lost interest spending more time at the club.
Salary has not been a problem to him all this while, except if he has needed an increment in the salary and the team probably have turned it down then it could be seen that it's very tangible reason for his exit from the team which I think is a very reasonable one. Another could be just like you rightly mentioned that a player would want to change their environment and wouldn't want to be in that same place they have been long enough anymore.
If a player is refund from moving at the time he want to move to a particular team, the chances that he will still be playing optimally as he should without the reservation on the fact that he's not playing in the right conditions where he should be playing as at the time when he's playing, it will be just best for him to leave and if need be with the clause the team may wish to add .