Everything is energy and the strongest of them is Love. If we are aware of it or not our energy is always affecting others around us. Animals and young children are much better at picking up other's energy. So if you can change your mood in the blink of an eye you can change your whole life.
When you are in down mood or a funk it is much harder to change. The first step in getting out of the bad mood is to just smile. If even that is too hard, take a pen or straw and put it sideways in your mouth. Your body will think that you are smiling and you should automatically feel a little better. If you still have trouble I suggest that you try meridian tapping.
The important thing to remember is that you always have a choice of how you feel. Others are just mirrors reflecting back to you the energy you are sending out. So, if you don't like what you receive, change your transmission to one of love.
This video explains it very nicely: