What is a friend?

I love my girlfriends. They are the people I trust completely and can relate to on all levels. They are amazing women, truly, yet I still seek company with males to broaden my perspectives on life. They give me balance, reality checks, and wonderful support. However lately I find myself at a loss with the definition of friend, for men. What are the boundaries? Why do they care about me? What makes him my friend in the first place? Will our friendship stop one day?
I care about these special people, but it seems lately their feelings appear to be disappearing. I don't know what's happened, sure people change but enough to grow apart from a person they once treasured? A person they felt totally at ease with, could share laughs, sorrows; reveal their inner dreams without any fear of judgement. I can recall so many times when these things came so naturally, and now it feels like it is a distant memory, a memory that sails onward toward the horizon. I feel so incredibly lost when I think about this thought, asking myself what could burn away a bond that seemed to be indestructible. I can't help but feel used at times, thinking that maybe the only reason they ever cared to get to know me was for personal amusement, that quickly turned to boredom. But what they don't see is that I am not a conquest. Our relationship is one that will never grow tiresome or old, instead it is one that has the beautiful possibility of lasting an eternity. Life is ever changing, and so are people, so why discard one that meant something special to you? The wonder of a friend is that they can be so many things in one body, a lover, a colleague, a classmate, even a soulmate. I know this is all part of a larger plan for us, and someday maybe it will make sense, but until then I suppose I will have to take a step away.