Ok, not a letter but a very interesting study….

Harvard mathematicians report an enormous likelihood that a marriage will succeed if a couple gives the same answers to just three questions: “Do you like horror movies?” “Have you ever traveled around another country alone?” and “Wouldn’t it be fun to chuck it all and go live on a sailboat?”

http://online.wsj.com/articles/new-ways-to-predict-which-marriages-will-succeed-1408636006?mod=e2fb