Why we get betrayed; Especially by Friends and Relatives?

Man is a social animal, it’s a hitting statement but the fact is, statement is true. We all love to live together, share our joys and sorrow together and it’s the feeling of togetherness or belongingness to our family, job, material things that give us energy to perform or do many tasks which we would have not done anyways. In positive terms we would not have accepted gruelling sales jobs or any other tiring sucking jobs or criminals would have taken that wrong path for earning to support their family needs.We all want to achieve something and for that we want to take help of others and make friend, relatives and many other undefined relations. The main objective is to reach a stage of comfort and satisfaction. But many a times we hate to support others for same objective of theirs.A bad situation is one when either we are not supported by our near and dear one. But The worst situation is one when we are get cheated by our these close circle of friends, relatives, colleagues etc. why people do so; when we don’t get answer we say he/she is an animal and it is his habit or nature.So these instincts are natural and unconditional to support someone's family of himself. We all believe in survival of fittest and we act in this way in extreme or choice situations. Animal instincts are also the same.I want to find some logics for such behaviours of these close ones. There must be some logic.....