Can someone please explains and bottoms to me?

This is a serious question: What makes a guy a total bottom? Or a total top? I know the obvious answer, but I mean what is the reason one would not switch from time to time? I can almost understand a total top more than a total bottom because of perceived pain problems but what makes one a total bottom? Does a total bottom have absolutely no desire to be a top or can he not get an erection in order to top?I also would understand it it were a hook up or VERY short term thing but if you are with a guy that has relationship potential wouldn't you want to do whatever needed to please someone you wanted to be with?It just seems that one would want to be a little more flexible to please their partner.Again, please don't think I am judging or anything, I Am just curious. I Am not the most knowledgeable when it comes to the gay community even though I am a member OF it. Maybe I am naive? Oblivious? I don't know, but I just wonder.It is a genuine curiosity of mine. Thanks for non harsh answers and opinions. And thanksEdit: Title should be "Can someone please explain tops and bottoms to me?"