I think is the fact that people are getting frustrated because they are grinding without getting much reward...
Not really because of the grind itself.....
The grind for reward for D2 is more much *rewarding*..
It makes you want to keep doing it.. like even if a run yield no reward...you know that the next run might drop somthing ....
..
You say to yourself.. 1 more run.. then next thing you know you're doing like 20~30 more runs.. !!!
But in D3 is like.. not as rewarding.. due to the fact the chance of a decent/great reward is much more rare...