We can trick our mind to become happy, long time back I read about this fact and since then I have tested it on me several times and find it working every single time. The theory says that if you wear a fake smile for a short period of time you will end up being genuinely happy.
In practice it involves smiling for about 5 minutes, I know it sounds funny and idiotic but the logic behind this, is that during that time the face muscles send the repeated signals to the mind that the person is happy and that is why he must be smiling. In the beginning, the mind rejects the signal because it remembers about the fake smile but over time the mind forgets about the fact and start believing that the person is happy(since he is smiling) and this makes the person genuinely happy. In the meantime, the mind also starts remembering the happy memories from the past and starts processing those images.
One thing to remember though is if anyone catches you smiling for no reason and that too for a period of 5 minutes he might suspect you of being crazy.