Low self esteem can be caused by so many things
The parenting you experience
how you fitted in at school
How much notice you take of things around you - ie media
how you interact with peers, friends
Sometimes its easy to fall into a pattern of being insecure. Someone says the origional thing to cause you to feel crap about yourself so you back away, then you are cautious getting close or giving the real you to someone else, and so you start to miss the emotional closesness etc, then you start to think that its because you dont deserve to be valued and it lowers you self esteem even further.
Its a case of breaking the cycle otherwise you will just go round in circles and it takes its affect on your family, work etc