We just butt heads a lot

NOTE: For the sake of keeping two of my very good friend's identity safe, the characters in this story are "Girl 1" and "Girl 2"

Me: "So are you living with *Girl 2* next year?" 

Girl 1: "Yea I think so, I'm just really afraid..."

Me: "Why? Aren't you best friends??"

Girl 1: "I don't know, we just butt heads a lot, it's not easy"

I could continue this conversation all day, but I'd rather not! In a nutshell, it just amazes and confuses me how despite being 'best friends' girls (tend) to treat friendships like light switches: turn them off and on whenever they want. Obviously this is just a generalization, but the other night when I asked one of my friends, who I consider to be one of the easy going people I know, if she is living with her best friend next yea (who I ALSO consider to be extremely easy going), there was so much hesitance. But why?? Is this environmental or genetic? I know I'm probably sounding like a sexist, but this is a common characteristic in almost all female friends I have. Why are my male friends not like this? In my limited experience, it seems like when guys have a falling out, it develops over time, however with girls there is so much boiling pressure underneath that it all just eventually explodes. Anyways, I just felt like throwing out the idea, and trying to comprehend why exactly this happens. One thing is for sure, no matter how hard I try to, I will never understand the complexity of female relationships!!