We can quite easily lose our self confidence, it often just seems to vanish without us knowing other than the fact it has gone. A number of reasons why this can happen is that through illness, unemployment, marriage or relationship breakdown, financial lack, health, etc. All of these things can play heavily on how we think about ourselves and our environment. Some people are naturally timid others quite the opposite, but aggressive people can often be the ones with little confidence so they make up for it with aggression.
Whatever you do keep your self respect in tact and never let anyone put you down.