But this begs the question - why did he pick James? Why not try to teach Sirius an important lesson in responsibility? Sirius sounds much like James - if not even more flippant - and might have really benefited from being placed in a position of leadership as well. So why James? Or why not Peter? Why not give him a lesson in leading instead of following? Why not prepare Remus for things to come, since he made it to the end?
Dumbledore being Dumbledore may have appointed James the position of Head Boy to try and teach James an important lesson in responsibility. Dumbledore must have used this lesson, as well as the position, to better prepare him for the things to come.