Since birth, we have been constantly learning. Though it is not something we "feel", we assimilate knowledge and build habits from new experiences daily. In the beginning, learning was effortless. As we grew into adulthood, the roots of our knowledge spread deeper becoming ingrained in us and making it more difficult to pick up new concepts.

This has nothing to do with our intelligence, we are simply creatures of habit... and not just those hard-to-break habits like nail biting. We unknowingly create habits for nearly everything. It's a matter of efficiency. 

Of course, we all wish to break ourselves of those unappealing habits. We design clever tricks to catch ourselves in the act--strings around fingers, random chimes every few minutes, etc. But building a better self is more than eliminating bad habits; it's about creating new, positive, fulfilling ones.