There's a version of this question that gets asked rhetorically, as a way of complimenting one group and dismissing the other. People who love the gym are disciplined; people who don't are making excuses.
The research on how people actually feel during exercise makes that framing hard to sustain. The differences are real, they're measurable, and a large portion of them are not about character at all.
There's a threshold, and it changes everything
The most useful single idea here comes from the affective response literature — specifically the dual-mode model developed by Panteleimon Ekkekakis and colleagues. It says roughly this:
- Below the ventilatory threshold — the intensity at which breathing starts to become laboured and lactate begins accumulating faster than it clears — affective responses are mostly positive, and they vary a lot between individuals. Cognitive factors dominate: your expectations, your self-efficacy, whether you chose to be there, what you're thinking about.
- Above that threshold, the picture collapses. Affect declines almost universally, and interoceptive signals — the body's internal reports of distress — start to override everything else. Your beliefs about exercise stop mattering very much.
- Right around the threshold, variability between people is at its maximum. This is where the genuine individual differences live.
Sit with what that implies. If somebody's entire experience of exercise has taken place above their ventilatory threshold — the school PE bleep test, a bootcamp class, a HIIT video, a well-meaning friend's session, or any of the "if you're not dying you're not trying" content that dominates the internet — then their belief that exercise is horrible is not a mindset problem. It's an accurate report of what they experienced. Nearly everyone feels bad up there.
How you feel during it predicts whether you come back
This would be a curiosity if affect were just a nice-to-have. It isn't. Affective response during exercise is one of the better predictors of future participation — in a number of studies it outperforms stated intentions, which are famously bad predictors of anything.
The mechanism is intuitive once stated. Behaviour is powerfully shaped by remembered experience, and memory of an experience is not an average — it's weighted heavily toward the most intense moment and the ending, the pattern Kahneman described as the peak-end rule. A session that finishes with you doubled over is encoded as a bad experience regardless of how you felt for the first forty minutes, and that encoding is what your brain consults next time the decision comes up.
People are not choosing rationally between health outcomes in twenty years and mild discomfort today. They're consulting a stored feeling. If the stored feeling is bad, the decision was made before conscious deliberation started.
The part that genuinely is individual
Some of the variation is dispositional, and it's worth being honest about that rather than pretending everyone starts equal.
Twin and family studies consistently find a heritable component to exercise participation — not to whether exercise works, but to the propensity to do it. Some of that appears to run through differences in reward sensitivity and dopaminergic signalling: the same session produces a stronger reinforcing signal in some people than others. There's also meaningful variation in trait tolerance of exertional discomfort, which is partly why identical training loads generate very different perceived effort across people.
And the "high" itself is more available to some than others. The endorphin story is largely folklore — endorphins are too large to cross the blood-brain barrier readily. The better-supported candidate is the endocannabinoid system, which does respond to sustained moderate-intensity exercise and does plausibly produce the mood effects people report. But it requires a reasonable duration at a reasonable intensity, which most people who hate exercise never reach, because they quit before the point where it would arrive.
Self-determination theory, and why lifting is different
The other large piece is motivational quality rather than quantity. Self-determination theory holds that durable, intrinsic motivation is built on three needs: autonomy (this is my choice, made on my terms), competence (I'm capable and improving), and relatedness (I belong here).
Autonomy alone explains a lot. Studies comparing self-selected intensity against imposed intensity find better affective responses at self-selected intensities — even when the actual workload ends up similar. Being told what to do at what pace is itself part of what makes it unpleasant.
Competence explains something specific about resistance training. It gives you unambiguous, frequent, personal feedback: the weight went up, the reps went up, the thing you couldn't do in January you can do in March. That's a competence signal delivered on a weekly cycle, indexed against yourself rather than anyone else. Running, for a lot of beginners, offers far less of this — the feedback is slower, vaguer, and much more easily read as failure.
Which is why "I hate exercise" and "I love lifting" coexist so often. They're not contradictory. The modalities differ in exactly the dimensions that predict enjoyment.
What follows from this
Start below the threshold, not above it. If someone is beginning from a standing start, the intensity that builds the habit is much lower than the intensity that looks impressive. You can add intensity to an established habit. You cannot add a habit to an experience someone dreads.
Give autonomy away. Self-selected pace, self-selected exercises, a genuine choice about which day. The adherence cost of a slightly suboptimal programme is far smaller than the adherence cost of a programme imposed on someone.
Engineer the ending. Because of the peak-end effect, how a session finishes gets disproportionate weight in the memory that decides next time. Finishing on something achievable rather than something brutal is a legitimate long-term strategy, not softness.
Chase competence early. Pick things where improvement is visible fast, and record it. The logbook is not just a training tool — it's a machine for manufacturing evidence of competence.
Stop treating enjoyment as optional. It's the strongest lever available on the only variable that ultimately matters, which is whether the person is still training in two years.
That's the natural next question, and the next piece takes it directly: given all this, what actually keeps people going?