There are many causes of low self esteem, but the leading cause of low self esteem comes when children are not given the positive feedback and love that they need at an early age.
When parents and other adults encourage children in their efforts when trying new things and guide them in a loving manner, these children will likely grow up with a healthy self esteem. This doesn't mean that little Jack or Jane must be told all the time that they're wonderful, smart and great looking, but that they're treated as individuals who are worthy of being held in high esteem.
This must ring true, however. Children know when someone is simply telling them what they want to hear. It's good for them to have to earn this esteem by trying new things. They also need to know that whether they succeed or fail in their efforts, they are still good people and deserve good things.
Children who are raised to think that other people are better than they are, smarter than they are or better looking, and just better, in general, will certainly have low self esteem. For them to have a healthy, high self esteem, they must hear that they are and be treated as if they're good people with a unique set of talents that no one else has.
A child may not be good in math, but very good in language, for instance. It won't do any good to tell the child he's bad at math--he or she already knows that. Instead, the child should be given positive reinforcement for what he's good at, language, and for trying hard to be good at math. This ensures that he gets the message that while he may not be good at one thing, he is good at another, and more important, that he or she should continue to work at becoming stronger in the areas where he's weak.
What children hear when they're young is what they'll tell themselves as an adult and this will determine low self esteem or high self esteem. Encourage and love children when they're small, forgive them when they fail but continue to get them to try to improve and experiment with new things and you can be sure the child will grow up to have healthy self esteem, not low self esteem.