Because I know I can't control tomorrow, so I don't worry about what will happen on me tomorrow, I just care about today. I am trying my best to live a happy life today. The most successful that I can imagine is you don't have regret even if you will die tomorrow, it proves that you successful life today.
Most of us always worry about what will happen on us tomorrow and what happened on us yesterday, in fact, all of these worry is useless. We can't change anything about it! So why spend more time on what we can control? Don't be silly anymore, okay?