If exercise can make you feel good, why do so many people hate doing it?

The evidence is clear: Exercise has mental health benefits, although studies have come to different conclusions about how large it might be.

And yet, when February rolls around again, the gyms suddenly aren’t so crowded as New Year’s resolutions lose their shine. Nationwide, only about 30% of people meet the federal guidelines for physical activity, and even in famously outdoorsy Colorado, just 35% do.

So if exercise gets the endorphins flowing and gives a quick mood boost, why do many people find it hard to stick with?