And not only people out there right now have said that. That wanting what you have is the key to happiness has been said for a long time. Here are two famous examples:

Don’t set your mind on things you don’t possess as if they were yours, but count the blessings you actually possess and think how much you would desire them if they weren’t already yours. But watch yourself, that you don’t value these things to the point of being troubled if you should lose them.

– Marcus Aurelius

The key to happiness is reduction of desires.

– Bhagavad Gita