1. Today is a new day, let go of anything bad (or embarassing!) that happened yesterday
2. Because... why not? Life is short, so be happy while you're still here!
3. There is too much violence and anger in this world. If you can at least try to be happy, it will make a world of difference and hopefully you'll pass the happiness on to someone else!
4. Because Pharrell said so: https://www.youtube.com/watch?v=y6Sxv-sUYtM