@tj4real. You're a master comedian, do you know that? I laughed my head off when you said:

Do some people marry their enemies?

Truth is that many people go into marriage without knowing what they are walking into. They assume a lot of things and do not reach firm agreements with their partners on critical issues. But after the ceremony the hard reality stares them in the face and, being unprepared to handle them, they begin to develop dislike and loss of faith in their partners.

The fastest way for a marriage to breakdown is when free, frank, and objective communication ceases.

Solution? Marry your friend and don't expect the person to get better; instead prepare to increase your love for the person - irrespective of who he/she later becomes.