All a Part of the Job

The main character's a Superhero, a Private Detective, a Monster Hunter, a Knight in Shining Armor, or the like. They're in a tough position. Their jobs are difficult. The hours are long and cut into the character's personal life. They work for somebody they despise. And to top it all off, their lives are always in danger.

They never get a moments rest. Every day they're called in for a new mission. And around every turn, some crook or monster is trying to kill them. They suffer injuries and put the lives of the people they love at risk.

So why don't they just quit? Because they love the job. To them, a little bit of danger is worth the satisfaction of a job well done. They love doing what they do, either because they enjoy helping others, or they enjoy the exercise, or enjoy the danger itself. Even if they don't like the danger, they realize that if they want to continue doing the job, the danger is just something they're going to have to deal with, because as long as they continue the job, there will always be danger. Not only do they manage to cope with this, sometimes they may actually love the danger, though it's not always necessary. When the danger is the primary reason to stay with the job, it's In Harm's Way.

Can also be justified with It's What I Do. Frequently says Think Nothing of It and Keep the Reward.

Anime and Manga
"Kotetsu: I’m a hero because I want to save people. Isn’t that enough?"
 * When Karina from Tiger and Bunny considers quitting her dangerous, stressful, and often thankless job as a Hero, she asks Kotetsu why he keeps at it (and puts everything he has into it, no less) when he doesn't even get the same respect for his work that the other Heroes do. His answer is simple.

Literature

 * In the Quiller novels about British secret agent Quiller, Quiller makes clear that he (and by extension the other agents) do their job because they need the excitement. He presents it as a not very sane defect. I mean, he is all for helping humanity and his country and doing the right thing, but if he goes too long between assignments he starts to hang around the office waiting (begging) for something and finds himself agreeing to take assignments that he would have otherwise turned down.
 * Kate Daniels works two jobs, one at the Mercenary's Guild killing magical nasties and one at the Order of Merciful Aid, keeping the peace and tracking down serial killers. She mourns that she can't even have a pet because she's never home often enough to keep it alive, she's perpetually in dismal financial circumstances, and she's constantly being injured, brushing up against death three times in three books. Later it's revealed that she was offered a job with the elite Red Guard for very good money, but turned it down because it's too boring to sit around and wait for someone else to attack you. She'd prefer to be out there hunting down the bad guy.
 * In Shanna Swendson's Once upon Stilettos, a gargoyle security guard's response when Katie thanks it for saving her.

Live Action TV
"Chief: Max, you realize you'll be facing every kind of danger imaginable. Max: And...loving it!"
 * Get Smart: Max doesn't appear to mind the constant danger his job entails. In fact, he relishes in it.

"Reese: (over the phone) Your life is in danger. Carter: I'm a cop. My life's always in danger."
 * The detectives on Law and Order series seem to fit this at times. Particularly Det. Stabler on SVU, who often finds his work emotionally unsettling and has had a lot of family problems because of it, but just won't quit for some reason.
 * On General Hospital, Dante Falconeri is an undercover cop who knows his job is dangerous but he is determined to capture a certain mob boss.
 * Easily applies to SG-1. First line of defense against alien invaders? Wouldn't have it any other way. Regular near-misses at getting Killed Off for Real? Not enough to make any of them consider resigning from the team.
 * Criminal Minds plays with this trope all the time. Justifiably, though, since everything about it applies: The characters are constantly being flown across the country and missing their families, they're constantly trying to convince mentally ill people not to shoot them (and sometimes have to kill said people or be killed), they're constantly being surrounded by mutilated bodies and grieving families, etc.
 * It's all in a day's work for Bicycle Repair Man.
 * Person of Interest: The POI is Detective Carter, Finch and Reese's Sympathetic Inspector Antagonist.

Western Animation

 * Darkwing Duck: "I knew the job was dangerous when I took it."
 * Also Super Chicken's Catch Phrase
 * Inspector Gadget is always on duty.

Webcomics

 * Xawu #23. "I was on the clock, actually" Complete with a Irregular Webcomic Shout-Out.