Originally Posted by kalniel
It's the same kind of thing that had people playing arcade games before personal computers became obtainable. You're in control of something and you have to act and think to overcome the challenge and survive to carry on playing. As you get further you come across new and harder challenges that require you have have improved your characters ability to face them (level, skills, runes, items etc.) along with new player techniques and skill. The successful overcoming of a challenge that you realistically thought you couldn't before increases dopamine levels and you feel more elated, just the same way crossing that gorge or climbing that ridge etc. makes you feel.
Games that fail to make you think you've actually overcome the challenge are not as fun, while well polished games of the likes Blizzard are known for, nail it.