When something is easy, we don't know what we did right. We just know that we got the desired outcome. 

This is good to do this specific task again. 

But it isn't so good to learn something else.

We learn the skill of learning by doing difficult things, or by coming close to succeeding yet failing to do something.

When something is difficult, we pay more attention, we look for ways to make it easier, and in the process, we learn how to identify areas for improvement and we learn how to get better.

We learn how to learn.

Doing something easy isn't practice. 

We practice difficult things so that they turn out to be easy when we actually need to do them.