What do you think when someone hearts their own comment or post?

If it's a debate/discussion and you are considered about the likes count, In such case, it should not affect you. You are not likely to change you opinions just because someone else or few people thinks in a particular way.

If they themselves liked their selfie or achievement post, either they are over excited, or not finding enough people to appreciate their selfie/achievements. It's never bad to appreciate someone and make them happy. If no one else appreciate/supports you, doing that on your own is not wrong either.