One of the most insightful lessons that I learned during my life is: Be cautious about the people whom you allow to enter your life (as roommate, as friend, or etc.). Some people can make you the happiest and the most successful person in this world and on the other hand, some people can convert your life to hell. It does not matter how hardworking, happy, successful you are, they can ruin your life.
I ruined my first year of PhD program by allowing a girl to enter my life (as ....)! Although I suffered a lot, Although I got very bad grades, Although she made last year the worst year in my entire life.... But, I gained an experience and this year I am so happy since I have very good roommate and very good friends around :)