We should only wear makeup for ourselves. Men's and everyone else's opinions are irrelevant.
From a self-empowering perspective, I understand why a lot of people have chosen to accept this. It's easy to identify with, provides a lot of self-reassurance, and it justifies subscribing to whatever the beauty industry serves us. Despite all that, I still can't find it in myself to wholeheartedly agree. There is so much more than meets the eye, especially if you bring feminism into the picture, so simplifying the relationship of women and makeup to something that assumes a similar situation for everyone is quite unfair.
Saying that you wear makeup for yourself and that everyone else should do the same thing is selfish, out of a lack for a better term, because not everyone has the same privileges you and I have. We constantly have to remind ourselves that there are actually people who have been indoctrinated to believe that they need to put makeup on to be taken seriously or be treated nicely. There are people who aren't lucky enough to know better, because what's important to them is they have enough to eat every day. There are people who have absolutely no idea how much their industries are exploiting their innocence to feed a mysoginistic and anti-poor machine. In short, even if we choose to deny it, not everyone buys and uses makeup because they want to and they love it.
When a 50-year-old woman uses anti-aging creams for her fine lines and concealers for her age spots, does she wear makeup for herself? Probably, but not entirely. A small part of her still believes that fine lines do not belong to a 50-year-old woman and that age spots are ugly and unwanted—these beliefs are fed to her by advertising and other ageist ideals. So perhaps when she finds satisfaction in using these products, it's because she has been programmed to follow these standards, and these products look like a step in the ideal direction.
Point is, everyone's situation is different. We can't assume that x does this because y, and that x should do this only for y. The world just doesn't work like that.
So those sales ladies in department stores whose makeup you make fun of all the time? Give them a break. They might not even like or care about makeup at all. For all you know, their funny-looking blush application was only forced by the company who pays for their salaries.