Reading Laura's answer, I have noticed that my question wasn't clear enough.

I have difficulties in making questions in English, so this is a good practice for me.

I think my questions should be made this way:

Do you think that people who are grateful to others are happier than people who are not?
or
Do you think that being grateful to others is only a matter of being polite?

Thanks, Laura for your answer, I agree with you.

I do not know if being grateful is related to happiness but when you are grateful for what you have, it means you value positively what you have or what you have been given (family, friends, a job, a hug in a bad day...)