In a world that is constantly changing there is no one subject or set of subjects that will serve you for the foreseeable future let alone for the rest of your life. The most important skill to acquire now is learning how to learn.
People love pretty much the same things best. A writer looking for subjects inquires not after what he loves best but after what he alone loves at all.
I try to write lyrics so that they won't age which sort of leaves you with the big subjects like death and love and sex and violence.