Success in teaching requires challenge! We humans, have an innate ability of problem solving; this is what got us from stone age to today's amazing technological world!
Simple doesn't mean easy! Simple refers to the wording and delivery of the content, while easy (or it's opposite, forming a challenge) refers to the level of learning.
Think of the feeling you experience, when you finally manage to achieve or solve a task or problem you have been working on for a week! It brings excitement, joy, confidence, it energizes you! It drives you to seek more posirive excitement of that level. Now think of solving a problem or task in a couple of minutes. You experience the same range of emotions, only they last for a second or two, and usually fade out on the spot.