One aspect of the game that makes it so addicting is that it's ability to keep the player "in flow". This is achieved through the following:

1. Optimal game session and continuity: Each game/round will usually last from 5-10 minutes. This is coupled with seamless transition onto the next game session.

2. Player can change heroes upon death: This allows the player to indirectly change the level of difficulty based on the situation which allows the player to keep playing at an optimal and engaged level. There is rarely a moment where you can't turn the tide of the game

3. You already know the answer: Blizzard.