Charlie DuChamp does not love his job.
He's never woken up with enthusiasm and joy at the idea of going into the office—not before his bad heart caught up to him and he was working Witness Protection, not after his heart caught up to him and he was pushing paper for Asset Forfeiture, and not now that he's lead in this experimental Fugitive Task Force.
He doesn't hate his job either. Not now anyway.
No, the only time he ever came close to hating his job was when he was stuck behind a desk. Some people are made for desk work, they have minds like Lowery's—primarily intellectual, dealing in facts and figures, with memories that easily store Denis Leary jokes and other trivial information that may or may not prove useful in the future. That's not to say Charlie (and those like him) isn't intelligent. But the part of him that works like that is stuck firmly in second place behind animal instinct honed through physical training and muscle memory that allow him to hit center mass on the gun range 99 percent of the time.
He needs to be out, chasing down leads, interrogating suspects, hunting. Lowery's type invented the bow and arrow so Charlie's type could use it.
He's not apathetic either. Though apathy is always there tempting him. Apathy makes a job like his easier. That ability to do the job, but go home feeling guiltless if they don't catch the guy right away. Apathy could stop the downhill slide he's put his marriage on.
It's not as if it's a job he got stuck doing, that he was forced into, or even that he was driven to do because of some profound experience. There are good people and there are bad people, and it's up to the former to keep the latter in line.
Actually, no. There are not good people and bad people. He works with good people (despite the fact that they are cons). More accurately put, there are people who obey laws and there are people who break them. Good and bad people fall under each category.
So, it isn't that he's "one of the good guys" that pushed him into law enforcement, into a job he neither loves nor hates, but is dedicated to.
It's more that he recognizes that society is like a machine. Not a well oiled, carefully maintained, beautiful machine—people may like the look of well kept classic cars, but for the vast majority they just aren't practical. No, society is a hard used, exhaust spewing, gas guzzling clunker that's so damn sour sometimes that Charlie's sure it is more of a lemon than all the citrus in Florida. But it keeps chugging on, trying to move forward, and every single person is a different part of that machine. Every single person has some function to perform that will keep the rust-bucket moving. And criminals…criminals impede that process. They work against the law abiding citizens that keep society afloat. Charlie sees this, and knows that, no matter how shitty the world seems, it needs to keep moving, and he's going to be one of the mechanics that keep it going. People don't deserve to stagnate on the dusty shoulder of life's potholed highway.
Perhaps he's taken the machine metaphor, too far. Really, it's probably more true that he doesn't give as much of a damn as the metaphor implies—he'd just likes to think he does. The heroic image of a man driven to save mankind from the delinquents of society may be cliché, but that doesn't make it unappealing.
It's also better than the real reason Charlie Duchamp is a US Marshal. Why he does a job he does not love.
He didn't do it to be a hero and save people, he doesn't keep doing it because he loves it, and he isn't so detached that he can live with failure.
He does it, first and foremost, because there is a need for someone to do it and there always will be. Criminals exist, therefore someone has to stop them. Whenever Charlie used to think about what he'd be good at, where he could make the most efficient use of his time, he always thought that there would always be criminals, so there would always be a need for people to bring them to justice. It was the sort of job that would never waste his time, and that he would never be left behind in, obsolete, unneeded. It's ridiculous and entirely human.
Charlie DuChamp needs to be needed therefore he is a US Marshal.