You can’t even talk! You’re 18 and you’re sometimes that way! Same person that always talk about the same damn things their own problems (moms abandonment) instead of thinking about others’ feelings. Also never ask how they’re doing! Remind YOU of anyone cousin? Some teenagers are like that but NOT all of them! QUIT GENERALIZING!!!
Teenagers like you and I aren’t fully developed! Don’t be calling out teenagers for that when YOUR doing that YOURSELF!

They are not. Just because the ones you know are like that, doesn't mean they all are. DON'T GENERALISE!

Not EVERY teenager is selfish.

I was fairly selfless as a teenager.

And just because somebody, regardless of age, may have a selfish moment or two in their lifetime, doesn't mean they're altogether selfish.

Teens are often trying to break out of the orbit of their parents & siblings to make / find their own identity/path.

With the coming of puberty, the changing bodies are sources of many things, including being insecure, and the attempt to find oneself while also trying to hide insecurity, etc., often leads to pretty horrible behavior. Add to that that the frontal cortex of the brain--where decisions are made--doesn't stop forming until around age 24, and you have a perfect storm of people with

hormones + rebellion + insecurity + poor decision-making + experimenting...

So you have to expect some bizarre crap.

That's my own observation, YMMV.

Teenagers will be teenagers

You were not teen in the 80's and 90's. It was really harder.