Difficult to Understand

It's difficult to understand the most important part of life i.e. friend.We cannot easily determine what a friend exactly want from us or what he/she try to convey.If we move close to friends,then might be some problem which makes us to remain away from each other.I don't exactly tell what may be the reasons which can damage the healthy friendship.Some reasons can be argument over small matter,ego,non-cooperation,not understanding each other.