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 March 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.

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

The popular perception that exercise produces endorphins, which then raise mood, isn’t wrong, but it oversimplifies complex interactions between brain chemistry and mental states, said Katrina Oselinsky, a post-doctoral researcher at the Health and Wellness Center on the University of Colorado’s Anschutz Medical Campus.

The same person could experience the same workout differently from day to day, depending on how they’re feeling, what else is happening in their lives and the setting where they exercise, she said.

“While endorphins are part of the picture, there’s a lot more going on,” Oselinsky said. “The same workout can feel energizing for one person and uncomfortable for another.”

While a subset of people get an immediate “runner’s high” in response to intense exercise, not everybody has the same experience, which can set others up for disappointment, said Emily Hemendinger, an assistant professor of psychiatry at CU Anschutz.

Brains differ in how sensitive they are to rewards, so some people are primed to get more or less enjoyment out of exercise — though, obviously, factors such as past experiences with exercise matter, she said.

Exercise does trigger a release of chemicals that give most people at least some sense of pleasure, but it doesn’t generate a tsunami that could drown out a person’s lack of comfort in their body, fear that others are judging them, perfectionism, boredom, physical pain or exhaustion, Hemendinger said.

“It’s not like doing drugs, and what that lights up in our brain right away,” she said.

For most of human history, exercise didn’t exist as a separate activity: We moved because we needed to find food or because we were doing something inherently rewarding, such as dancing, said Marcelo Bigliassi, an assistant professor at Florida International University.

Now, people need to find a motivation for exercise, which isn’t always readily apparent, he said.

“We don’t do anything without a reason,” Bigliassi said.

While the brain releases dopamine, triggering the desire to repeat exercise, it can also get “really creative” in coming up with ways to avoid something uncomfortable, Bigliassi said. To get around that, people need to focus on finding something challenging enough to give them a sense of accomplishment, but not so tough that they give up, he said.

“We don’t want people to find something they don’t like, exercise at high intensity, probably injure themselves and … feel like a failure,” he said.

The data is clear that, over time, exercise improves both mental health and cognitive abilities such as problem-solving, Bigliassi said. But many people give up long before they see those benefits because they set themselves up to fail or aren’t prepared for bumps along the way, he said.

“Consistency is much more important than everything else,” he said.

People who believe they’ll be able to accomplish what they’re trying to do report more enjoyment from a workout than those who feel defeated at the start, Oselinsky said. Having support from the people around you also improves the experience, and new exercisers tend to develop more positive feelings as they get better at the activity, she said.

Of course, we humans aren’t always great at sticking to something we don’t like, even if we know it brings benefits. So a non-exerciser’s best bet is to choose an activity they like or to bundle exercise with something else they enjoy, such as getting outside, spending time with a friend or pet, or listening to music or podcasts, Oselinsky said.

“If you’re trying to force yourself to stick with something you don’t like, it’s going to be harder,” she said.

People often feel that they have to go to a gym, but movement throughout the day can be as good as a dedicated exercise session, Oselinsky said. That could look like doing physical labor at work, schlepping toddlers whose motivation to walk is shaky, or tending a backyard garden come spring.

Federal guidelines recommend 150 minutes of moderate activity or 75 minutes of vigorous activity each week, with some sort of muscle-strengthening exercise on two days.

But any movement that someone enjoys is a step in the right direction, whether that means jumping on a trampoline or playing with their dog, Hemendinger said.

“Joyful movement’s hard to find, but it’s important,” she said.