What is it about running that keeps many of us coming back? From an evolutionary perspective, it is actually a strange question. Daniel Lieberman has written extensively that humans did not evolve to exercise simply because exercise might make us healthier thirty years from now. For most of human history, physical activity had a purpose. We walked to find food, carried things, hunted, gathered, played, escaped danger, or traveled somewhere we needed to go. When there was no reason to expend energy, conserving it made sense. So why would millions of us voluntarily wake up early, put on running shoes, and spend an hour burning energy we do not need to burn? Part of the answer may be that running eventually starts rewarding us for doing something evolution ordinarily tells us to avoid. Moderate endurance exercise increases endocannabinoid signaling, including anandamide, which is associated with reduced anxiety, less pain perception, and the calm or mild euphoria runners often describe afterward. The old explanation for the “runner’s high” focused mostly on endorphins, but newer work suggests the endocannabinoid system may play a larger role than we once thought. That reward probably matters because the exercise itself is not always pleasant. The first mile can feel terrible. Hills hurt. Some runs are slow, awkward, or miserable from beginning to end. Yet many runners notice that they feel different afterward: calmer, less anxious, clearer, and somehow reset. On average, acute exercise improves mood, though not everyone experiences it to the same degree. But I doubt the chemistry is the whole story. Running eventually becomes familiar. There is the rhythm of your breathing, the repetition of your stride, the road or trail you know by memory, and the strange narrowing of attention that can happen after twenty or thirty minutes when the noise of the day begins to recede. You are doing something physically difficult, but mentally there is often less going on rather than more. Maybe that is part of the paradox. We may not have evolved to exercise for exercise's sake, but we did evolve to move, explore, endure, compete, and occasionally cover long distances. Running may take some of those ancient systems and place them in a modern activity with no obvious survival purpose. Maybe that is why some of us keep coming back. Not because every run feels good, and not because we are somehow more disciplined than people who dislike exercise, but because over time our brains begin attaching reward, familiarity, and meaning to something that initially looks like unnecessary work. Evolution may have taught us to conserve energy. Running may have taught some of us that spending it can feel pretty good.