Isn't it funny how quickly our emotions can change? It's like emotions are just waiting to get us when we least expect it. One minute you can be happy and then sadness can just smack you round the face out of nowhere.
One exercise to gauge how your emotions change is to keep a mood diary. It might be entertaining, enlightening, even insightful. A good way to see who or what changes your mood on a day to day basis. That way, you can see what changes you need to make and keep those emotions in check. Once you understand them and know your triggers, you can understand yourself a lot better...if you want to of course!