To be happy, you have to find out what took the smile off your face. Most importantly, before working on what causes you to frown, understand yourself. Get rid of the conflicts about who you are, instead, understand your strengths, weaknesses and accept them. Being honest with yourself is the starting point of unadulterated happiness.
Sadness is caused by most reasons, just to mention a few it could be ; your partner left you, you are over grieving a friend or a family member, you are afraid your friends aren't enough, you moved in to a new place and you are yet to adjust, you don't like certain things about yourself.