The Thinking Behind The Tweet
But, Difficult should not mean Unpleasant.
Physically-difficult work burns up energy in muscles, etc. It strengthens the muscles...that's a good thing...unless you're lazy, in which case physically-difficult work would be annoying.
Cognitively-difficult work burns up energy in neurons and other places. It strengthens neuronal connections...that's a good thing...unless you're lazy...then work, again, would become annoying.
About being annoyed or stressed out, etc: the cause behind negative feelings is not work. So, let's not place the blame on work when, in fact, there is another culprit.