Happiness within yourself...is a mind boggler. lol!
Usually we find joy in what we do. This keeps us happy.
Someone who likes making furniture, finds joy in the process. This keeps him happy. If someone comes to him and asks him to make a chair because he likes his work, then the furniture maker gets to share his joy with the person who wants a chair from him.
When the chair is made, and the person uses the chair...and the furiture maker knows that the chair is being used... Everyone is happy.
....until the guy who bought the chair doesn't pay... Then its the lawyers who are happy.