I don't.
a) can't be bothered
b) I must think I look ok, I suppose, which must make me either vain, or deluded, or both...
c) I don't really care what other people think if I don't wear it - i.e. I genuinely wouldn't judge someone either way, and would be utterly bemused if someone judged me for it
d) like another poster, the last time I wore it I felt self-conscious, I guess. Not because I have self-esteem issues, I think, but just too, uhh, "showy", if that's the right way to express it. I like to look good in nice clothes, etc., but I don't particularly want to draw attention to myself in a "look at me" sort of way. I'm happy for those who notice me to do so naturally, if you like. Wearing make-up, even light make-up, felt like too much of a banner sign of "Here I am, trying to be seen". Wearing make-up is inherently part of an attempt to conform to a societal consensus about what constitutes the attractive or beautiful. Nothing wrong with that. Plenty of things we do that are culturally normative, etc. However, for me, to try to make myself beautiful if I am not (and again, I think I look ok, but I'm no Natalie Portman) just feels like such an effort, and such a delusion. Etc. Hmm, not expressing that well, but it's late and I should be doing other things... I'm not saying that this is how it feels for people who wear make-up every day, btw, just how I felt.
Actually, I've just realised what I want to say: I find that - for me - wearing make-up isn't "making the best of myself" (or really, in my opinion, of anyone, though if people want to wear it, that's cool), it's always already falling short of the societal norm of beauty that we're being sold. Unless you are beautiful already, in which case you probably don't need make-up in the first place...
Phew. God. Really must work now...