Because your today determines your tomorrow.

If you spend your day worrying about tomorrow,

Then the next day you would do the same...worrying about the future.

At the end what do you get? Nothing.

Rather fo...

"Do not worry about tomorrow, for tomorrow will worry about itself. Each day has enough trouble of its own." This means that you should focus on enjoying the present moment and not worrying about the future.

Screw the what-ifs.

For example, replace, in your relationship with your ex, worrying about your ego and what's fair and keeping score with love (for example, say 'ok, that's what you think and that's just fine').