I don't think the cause is fully understood.  It could be genetic, could be biological, or it could be psychological.
I have quite a lot of energy, but I know some people just aren't as lucky. Is their a medical condition for being constantly tired? There is a condition called chronic fatigue syndrome in which people have long-term fatigue in a way that limits daily activity. That must be awful. I wonder what causes it. Depression perhaps?
Biological, genetic, infectious, and psychological mechanisms have been proposed, but the cause is not understood.