What happened to me today happens on a fairly regular basis. I was irritable, quick to tire, in no mood to talk, slightly depressed and generally uncomfortable with myself. When these symptoms strike, i've come to recognize them as signs that i'm experiencing "that time of month". Yes, that's right - i think i have the male version of PMS (referred to by researchers as "IMS" - Irritable Male Syndrome).
Blame it on PMS
Laugh if you want, but research (also, see Jean Lawrence for further reading on the topic) has confirmed that women are not the only human species to be affected by regular hormonal changes in their bodies.
I guess the main difference is that the changes in male behaviour tend to be less noticeable. Most men are normally aggressive (this is a relative judgement and is a pure generalization) and this may disguise any indication of a temporary hormonal change leading to a slight shift in behaviour. It could also be that the change in behaviour is often so minimal as to be difficult to detect as a change at all.
So the next time you see a slight change in the behaviour of a male someone you know, take a moment to consider the possible reasons.