Noticing Default Moods

I have noticed that when I am at work my mood changes quite drastically. I could be having the best of days and then as soon as it is time for me to go to work I'm not in such a good mood and it is interesting because as soon as I leave work I'm not as stressed or in a bad mood. My job stresses me out a lot to say the least, but in terms of default moods I automatically associate work with an unpleasant mood. It immediately triggers a default mood that I only feel when I'm at work.