what decides our beliefs?

How we make an opinion about a thing?
At the beginning,we let our mind decide and after we arrive at a conclusion,we associate number of reasons to support our belief.We become orthodox,we become blind.We make our mind think in unidirectional way.And if somehow something changes our belief,we soon adapt ourself to that changed belief.The girl for whom i was the best person in the world now have all reasons to believe i am the worst man alive.