When we go to the gym to get some exercise, we accept that, to get any bang for our exercise buck, we need to feel like we are working hard. You want to break a sweat, maybe feel a little sore later, struggle with the last repetitions in a set, and breathe hard. To make your body stronger and more fit, we accept that we need to work. Hence, we call this process - working out.
Likewise, if you ask too much of a gym workout - you either won't succeed (you can't lift the whole stack without working up to it). Alternatively, you may finish it but be too sore to follow through and go back to the gym the next time.
It seems to me that learning is the same process. So, it shouldn't surprise us that to make gains in learning and understanding new ideas (algebra, for example)... we should feel like we are struggling a little. Wrapping our minds around something new doesn't come easily. So, if you aren't feeling like you are struggling a bit - you probably aren't making gains. Obviously, if you can march through your schoolwork without really paying attention to it - it is too easy and you won't make gains. Similarly, if it is too hard - you won't be able to struggle to the answer - no gains again. Thus, one real key to education seems to be finding the "sweet spot", just the right level of difficulty.
Another key to education, which can be very difficult to develop, is a willingness to struggle. We intuitively want things to come easy and make us feel smart. But, to really figure something out, some degree of struggle seems to be necessary. If we frustrate too easily, we will miss these new insights.