Aggressive

Question
I have a four year old male boxer and just recently he has shown aggressive behavior.  He is an extremely playful, loving dog, but recently he bit my roommate.  
His first sign of aggressive behavior was after hiding under the bed.  He loves "cave-like" areas and would sleep under my bed.  One morning after trying to get him out he growled and snapped at me.
Just the other day he was trying to smell another smaller dog under a park bench and when my roommate reached under to get him he bit her hand twice.  
I have never seen this aggressive behavior until the bed incident and now it seems to be about once a month he has a spell and will either growl and snap or actually bite.  
HELP!
I cannot imagine life without him. Is he being territorial, is he not feeling well??

Answer
I would definitely have a medical workup done, and be sure to have a full thyroid panel run ("OFA panel") - often thyroid or other health problems present as aggression.  

That said, reaching for a dog who is in a 'safe' place is generally seen as an aggressive move on your part; his reaction was probably less aggression than fear.  

This site has lots of information about aggression, including possible medical causes:
http://www.k9aggression.com/

I also recommend the book, "Aggression in Dogs" by Brenda Aloff.

Good luck!