Could someone explain me how friendship works in astrology?

I have a friend who is born just about two weeks later than me, so the same planets pretty much in the same places.

But sometimes she just makes me so angry that I don't know why we shoud be friends. 

Why is that? Shouldn't we understand each other very well?

Aquarius mercuries conjunct except hers is retrograde ...

well, it's not exactly astrology, but i find that with the folks close to me whom have lots of similarities with me (like my daughter), when i get the most frazzled it's when she's exhibiting something i share but don't like about myself. i chalk it up to projection issues. but have no idea if that applies to you.