The secret to happiness is/is be/: be/is to be glad for …

[Odessa Dawn]

The secret to happiness is to be glad to what you have …
The secret to happiness is be glad to what you have …
The secret to happiness: be glad to what you have …
The secret to happiness, be glad to what you have …
The secret to happiness is to be glad for what you have …

May I ask if those phraseologies would ever be written by English users as given?

 

[bhaisahab]

I don't find any of them natural.

 

[Odessa Dawn]

Thank you. Then, what is the best way to make either natural, please?

 

[bhaisahab]

"The secret of happiness is being content with what you have." This is possible.

 

[emsr2d2]

The secret to happiness is being happy with what you've got/with what you have.
The secret to happiness is to be happy with what you've got/with what you have.

Edit: Interesting, I just noticed that bhaisahab used "The secret of happiness" and I used "The secret to happiness".