Because
every second you spend being angry or sad is a second of happiness
you can never get back.
Believe in yourself! Have faith in your abilities! Without a humble but reasonable confidence in your own powers you cannot be successful or happy.
I am determined to be cheerful and happy in whatever situation I may find myself. For I have learned that the greater part of our misery or unhappiness is determined not by our circumstance but by our disposition.
I, not events, have the power to make me happy or unhappy today. I can choose which it shall be. Yesterday is dead, tomorrow hasn't arrived yet. I have just one day, today, and I'm going to be happy in it.
Be happy with what you have and are, be generous with both, and you won't have to hunt...