Dopamine spot is the opposite of pain point, a gameplay graph in a great game would contain many dopamine spots and few pain points.
Generally it is a state of relief/reward/success when player accomplishes something valuable in-game.
The event doesn't have to be exceptional as in-game reward/success, however typically the event has to be rare, novel or exclusive enough at the player current state.

Type of dopamine spots:
1.Material rewards: valuable items/objects suddenly found by player.
2.Gameplay Accomplishment:gaining levels,quest rewards, pvp wins.
3.Economic: Player feeling influence on game economy.
4.Social:Players increasing their reputation/fame.
5.Skill:Players finding another optimal playstyle/skill/item/character build. Creating a new playstyle or tactics also counts.
6.Novelty:Players discovering something new or unusual(including in-game glitches/bugs).
7.Exclusivity: Players belonging to exclusive group or elite few(rarest gear, first to discover/accomplish something).

The brain self-regulates: having too much of reward for little effort will reduce its value and make it "normal" and less novel to encounter again(such as boss farming).