why does it seem girls only think about themselves?

Answer 1

This is not a male - female question, it is a human one.
People think about themselves first, if they have no one to look out for them.
The ideal relationship becomes, when 2 people come together, that are more concerned about the other than about themselves.
And that is what we are all looking for, but very few of us are lucky enough to find it.

Answer 2

because we can only depend on ourselves