The cause isn't understood, but several different mechanisms have been proposed.
It can be biological or infections or genetic. Tests must be performed to find out the cause.
My friend has chronic fatigue syndrome.  Its sad That's too bad. I'm aware that this limits a person's ability to do daily activities. Yes.  He has a lot of trouble doing even everyday things in life
Biological, genetic, infectious, and psychological mechanisms have been proposed, but the cause is not understood.