Some runners cross the Boston Marathon’s finish line with hands held high, a look of elation on their faces. Others find themselves slumped in a medical tent with Jeff Brown, lead psychologist for the Boston Marathon medical team.

“We’re not talking about, ‘Oh, I need ice for an ankle,’” Brown said about these finishers. “Someone is significantly overheated or underheated. They’re having terrible cramps. They’re disoriented. They might not know exactly where they are.” Laid out on cots are people with extremely low levels of salt in the blood, and others who are sad, fearful, and agitated for reasons they can’t explain. Brown’s role, along with his team of mental health clinicians, is to help perform psychological evaluations and recognize symptoms of a wide range of medical conditions.

Seeing these high levels of acute distress mere meters from the finish line, some might ask, “Why bother?” There are other ways to stay in shape or raise money that don’t require an extreme feat of cardiovascular and muscular endurance over multiple hours in unpredictable weather conditions.

It’s a question that Brown, a Harvard Medical School lecturer, McLean Hospital psychologist, and author of “The Runner’s Brain,” ponders each year as thousands of runners funnel past him. It will no doubt be on his mind Monday during the 129th edition of the Boston Marathon.

The reasons, Brown said, are inexhaustible, but what they have in common is that they’re “very, very personal, and really it is that personal energy and commitment that keeps people going, regardless of where they are in their lives.”

Over the years, he’s met hundreds of people who are running for a recently deceased loved one, contending with a cancer diagnosis, and fundraising for a beloved charity. He’s met women who — monitored by medical staff — finished the marathon while far along in their pregnancies and other athletes who explicitly ignored their doctors’ instructions and ran with cracked femurs, torn muscles, recent sprains, and diabetic complications. “Perhaps it’s not a surprise,” said Brown, “that they meet us in the medical tent at some point.”

A marathon channels people’s energy into a methodical, focused pursuit, and, especially at Boston, one that provides some bragging rights. “It allows people to come to terms with themselves,” Brown said. “When it comes to self-concept and belief about one’s capabilities, we always do better when we have some sort of objective measure.”

That objective nature is crucial, Brown says. Not only do you complete a race, but when you finish, you get a medal placed around your neck. “I think of that as kind of this transformational moment,” he said, “because it’s something that was a hope that is now realized as a wish fulfilled. It’s the mind-body thing happening.”

He loves seeing people he’s treated gather enough mental and physical strength to leave the medical tent and finally collect their medals. “It’s almost like they had a chance to review their whole experience one more time,” he said, “and it might mean a little more to them.” He’s seen huge smiles, tears, and quiet reflection. “I think that’s just a reflection of the vast continuum of emotion and purpose and goals that people bring to running the Boston Marathon.”

Marathon runners invest enormously varying amounts of time and energy preparing for the race. Some are young, single people who sacrificed late nights out and lazy weekend mornings to set a personal record. Others are older, first-time runners who might be taking time away from their kids and spouses to complete a bucket-list item. A few are looking to advance professional running careers, and others show up having done barely any training at all.

A medal — and some bragging rights — are far from the only reward that motivates some people to invest thousands of hours into race-specific training and for others to ignore the sound medical advice of their doctors.

“In our world that’s rather cluttered with a lot of criticism, it’s a really nice way of getting affirmations in a healthy way,” Brown said. “And people, in our heart of hearts, we just want to be treated civilly.”

Running is also an opportunity to change your own conception of yourself and, at least for a few hours, how others view you. “For a while there after you complete a marathon, you’re kind of a hero,” Brown said. “You’ve done something that a lot of other people would never set out to do or think about doing, which is pretty darn cool.”

The mental side of running still fascinates Brown and has kept him on the Boston Marathon’s medical staff for more than 20 years.

“That one day, with 30,000 runners, there are 30,000 different ways of completing that marathon,” Brown said. “Imagine all the thinking and psychological experiences and reflections and motivations and negative thoughts and positive thoughts that went all those 30,000 different ways.”