But like anyone else, they can have mood swings at any time for all sorts of reasons. Teenagers are often thought to be especially "moody", which is easily explained by the emotional and hormonal turmoil of adolescence.

However, if someone has extreme mood swings, that might be a sign of a problem such as depression or bipolar disorder. If your friend is suffering because of his mood swings, it might be a good idea for him to talk to a counsellor.