I've taken selfies before (granted, it was when I was 18 or so) so I may be able to help you understand them, lol
Often selfies happen because people want to appear physically attractive and/or post a photo of them that shows off how attractive they are. Even if they aren't perfect 10s, they want to showcase their best features.. That's why you'll notice that nobody posts selfies of themselves on the toilet or with a face covered in food. The few times I posted selfies it was because I looked good, wanted people to see, but wasn't about to go to any event that would warrant having someone else take a pic of me. In retrospect it was a little desperate, but that's another reason why it's so common with kids. I never understand when people in their 30s do it.