Guy mood swings.

I work with a guy, he used to be very pleasant always smiling and happy, that was about twelve months ago but over some period of time he has changed. He has become more quiet (he's shy anyway) and he has days where it seems he's angry or upset and moody and just not the person he is. He seems more distant now, but he seems better dependant on who he is around sometimes. He has days where he is great, smiles, talks and laughing then the next day he will be totally diferent. He will come into work, with his head down avoiding eye contact and just seem quiet, moody and cold.
I don't know what to think? Am I taking it to personal?
He can be quite withdrawn sometimes so he seems difficlut to approach, plus I'm also shy
Any advice? What's wrong with him?

I work with a guy, he used to be very pleasant always smiling and happy, that was about twelve months ago but over some period of time he has changed. He has become more quiet (he's shy anyway) and he has days where it seems he's angry or upset and moody and just not the person he is. He seems more distant now, but he seems better dependant on who he is around sometimes. He has days where he is great, smiles, talks and laughing then the next day he will be totally diferent. He will come into work, with his head down avoiding eye contact and just seem quiet, moody and cold.
I don't know what to think? Am I taking it to personal?
He can be quite withdrawn sometimes so he seems difficlut to approach, plus I'm also shy
Any advice? What's wrong with him?

Maybe he has low self confidence? Maybe he has mild bipolar? Maybe he has difficulties at home? Maybe he just has bad days sometimes? I wouldn't take it personally, all sorts of things can affect someone's mood, maybe ask him if he's okay and if he says 'yeah' and leaves it then I wouldn't press matters any further

He is not the most confident guy, he stares a lot, and he seems more cold with me, but maybe I'm looking into it too much? Even when he seems cold and moody, he's still polite, thanks me etc, and when I thank him, he says 'you're welcome'

I know this'll be an odd suggestion for why he might be behaving like this but he could be taking some medication leading to such symptoms. It will seem like a wild suggestion but some medicines have extreme side effects on mood and temperament; specially if they are used for an ongoing treatment.

Look up Roaccutane as reference.