Happiness is like a butterfly - Henry David Thoreau

Don’t chase happiness, it will come by itself when you least expect it.

You could also change the word happiness for love and the same would be true.

If you chase love, it will run away from you.
If you continue with your life without the desperation of looking for love (or happiness), you’ll find it will eventually come along when you least expect it.

Happiness is like a butterfly.
The more you chase it, the more it eludes you.
But if you turn your attention to other things,
It comes and sits softly on your shoulder.
– Henry David Thoreau