There are two things about a smile that's worth drawing a lesson from, and together I think they teach a priceless principle.
First, it takes less strength to smile than to be sad.
Second, it's easy to forget. Yes, most of us simply forget to smile, myself included.
What can we learn from these two combined? Ironically, the easiest path requires the greatest and most constant effort (i.e. long term, of course).
Antithesis: The toughest path is to pursue a course of least resistance.
So, I suggest, "easy is harder", regardless of how we define "easy". Why? If we think of easy as a short-cut or a form of instant gratification, the consequences will surely punish us around the next bend. If, on the other hand, by easy we mean long term smartness, of necessity we pay a hard price in the moment.