For me it's having thoughts running all over your mind about the things you like or love Like for example if I love the work I do, the thoughts in my mind would be related to that work and I would obviously be in happy state . If I have to do something I don't like then thoughts running through my mind will be related to that work and I won't be happy. I love trains so watching a train crossing or parallely driving on a road adjacent to a train is like pure happiness