Thank you for posting this thread as I love this topic.
For me, it's food, drinks, some songs, exercise (usually after and not during) and sex/ massage/ romance. In short, gratification and they are fairly easy to get.
Also, when a problem is solved, a task done, a mission accomplished. Yay! These are usually challenging.
And, when all is forgiven and we moved on. Awww These are situations that involve hurt/ anger, etc.