Why do some people fall in love with a fellow of opposite gender?

I have come across many friends/ members here who admit that they have fallen in love with a member from opposite gender and then the got married to the same person. I wanted those friends/members to share with me the secret of falling in love, what makes them fall in love with a person of opposite gender, what plays in their minds, when they realize that they have fallen in love and what are two most important traits/qualities of the other fellow which prompted them to fall in love with him/her. Many thanks in advance for your likely responses.