Oncoming sickness, particularly if the unwellness translates to a general feeling of malaise. [Refer to: does no trust your judgement. ...But okay, fine, he'll be helpful:]
There is also the possibility of dehydration, malnutrition, or lack of sleep. It might be something he has consumed? The human state of mind affects the physical well-being as well, so there could be other feelings at play rather than merely sadness. [Marvel at his exertise on human health.]