What makes us trust others? Is there always some solid reason behind it or its simply our own need that ?

Whether or not you trust others is based on your past experience with people. If you have had people backstab you in the past, you will probably have a cynical view on human nature and not trust anyone. If you have had many kind, nurturing, fair people treat you with honesty in the past, you will probably be quicker to trust people. One thing I have noticed about trust though: It is alot easier to lose someone's trust than to gain it.