19 year old not wanting a birthday party?

My 19-year-old son does not want to do anything to celebrate his birthday. He doesn't want to have a party with his friends, not a party with his family, he doesn't even want to go on a mini-vacation.He just wants to be left alone on that day. What are the possible reasons for such behavior? Is there... show more My 19-year-old son does not want to do anything to celebrate his birthday. He doesn't want to have a party with his friends, not a party with his family, he doesn't even want to go on a mini-vacation.He just wants to be left alone on that day.

What are the possible reasons for such behavior? Is there anything I can do to help him? Any help is much appreciated.
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