It's kind of hard to be cheerful first thing in the morning, so don't worry if someone snaps your head off for no apparent reason today. You can't let your mood be affected by someone's else's, so try to let it just roll off your back. Focus on the rest of your day and leave this grumpy person behind as you move forward. Getting outside at some point today will also help keep your mood high: the fresh air will help you think more clearly and see things in the proper perspective.