I think the best reasons are because it feels SO good and no one can do you like you can do yourself. But also because it's a GREAT way of relieving stress and tension and it's one of the best and certainly the only 100% safe way of sexual gratification outside of sexual relations with another person. I think it's healthy, natural and good . . . so long as it isn't done because you have nothing better to do (as someone here said), or as a way of constant escape instead of dealing with your reality, whatever that is.