I run and cycle a lot--most weeks I spend at least six hours traveling by foot or bicycle. Too often, I encounter runners looking completely miserable. The other day I passed a woman running the other direction. Her head was down, her ear buds were in, and her face was contorted into a painful grimace.
If you're going to exercise, why not enjoy yourself? It baffles me to see people outside, in beautiful weather, looking like they're getting a root canal.
Running should be fun. If you're not having fun, why do it? While I'm out for a run, seeing another runner smiling, nodding their head in my direction, and saying, "hi," always makes my run more enjoyable. Positive energy is contagious.
Sharing that joy of motion while running is just one benefit of smiling while you run.
Researchers have found that people who smile more often are happier. Does this mean that happy people smile, or that smiling itself makes you happy? Scientists have found that it goes both ways. They've also found that people experience less pain when they're smiling (read this article from Scientific American).
So when you're on your next run, bike ride, or walk through the park, smile. If you don't make someone else's travels more enjoyable, you just might enjoy what you're doing more.