1) Realizing that you are alone in this world. Maybe you do not have anybody to talk to, to share your secrets, your impressions. You do not have anything in common with anybody.
2) Unhappy love. You fancy somebody very much, but he/ she does not answer your feelings or even does not know about them.
3) Lack of money. You classmates wear stylish clothes and you have to wear old, unfashionable things because your parents are not able to afford to buy you new ones.
4) Some religious statements.