If something is too hard or too easy, you learn nothing.
I cannot learn to mountain-climb by starting with a solo summit of Everest. In fact, I will die a horrible death.
But I also cannot learn to mountain-climb by just walking up my slightly-sloped front lawn over and over again. I will learn nothing.
Too hard is the danger zone. Too easy is the comfort zone. But juuuuuuust right is the learning zone.
But really, that just means that too easy and too hard are both the danger zone. Because few things are more dangerous than coasting through life without learning anything.
Remember: all self-improvement is uncomfortable. You are shedding weakness and ignorance and failure; extracting those things from yourself is not painless. If you’re looking for comfortable learning, you’re looking for entertainment. That has it’s place, but don’t think you can improve your life from it.