The very nature of the service industry means that our lives are often interrupted by the needs of others. Customers need kindness and understanding especially if they have an emergency situation. Instead of feeling resentment from being bothered, learn to deal with the wants and needs of customers and treat them as opportunities. Some of us have trouble recognizing the fact that service means to take on the role of a servant.
Our day may open with harsh new problems or a daunting work schedule. A happy disposition may take a bit of effort because this kind of day does not invite a cherry mood. We need to do a better job of seeming cherry even when we don't feel that way. It has been said you should "fake it until you make it." Many times when you go through the motions of happiness, the reality of happiness clicks in not long after.
Look happy. Act happy. Be happy. Perform happy. All this should be completed with enthusiasm. The results will be rewarding.