DIfferent people define happiness differently.
Some of it maybe because personal experience differs, and for every person and situation there are different aspects of life people concentrate about. And different ways people think about themselves.
It can be rational for some to concentrate on pleasures of the mind. If that is where they can find happiness or where they feel comfortable about.
Others feel comfortable only alone. They concentrate of these things.
Others concentrate on doing the right thing.
These tendencies may be mistaken or right. But they maybe practically the right way to manage life and getting the best of it for the individual person.
PS. Much more subtleties can be found in details of emotional attention, and various aspects of self and experiences people do have.