Defective Detective
Dan: You're talking to yourself, Michael.
Raines: Dan, um, I appreciate the concern, but do you ever think that was maybe just my hook?
Dan: Your hook?
Raines: Yeah. I'm a, uh, a crazy detective. Look out, he's nuts! Wheee!
— Raines, "Meet Juan Doe"
The greatest challenges a detective faces aren't always a devious criminal or a really tough case — all those are a cakewalk compared to managing their personal life.
The genius ones are nerds with trouble getting along with people or worse, have anti-social or personality disorders. The hard-working ones are workaholics who let their family relationships slide because they're never home. The overworked and nervous ones dabble in drugs and court substance addictions (or blood.) The Film Noir detective and his descendants have terrible luck with women, who either end up dead, broken or distant; if he has a wife he may be cheating on her. And gods help him and his friends if some of the bad guys or associates that they helped put in the clink come back to haunt him.
In short, it's rare to have a detective as a main character in a dramatic story and have them not have at least one serious character flaw that's tangential to them actually working cases.