Why Did the Amateur Sleuth Cross the Road?

Unless an author writes only police procedurals and P.I. novels, the question of why the protagonist gets involved in the mystery is present. I've never in my life wanted to investigate a murder, and I'm guessing most readers would say the same. We trust the police to do an adequate job, and while we might grumble that they didn't turn every stone in a specific case, we seldom jump in to help.

If you're going to write a mystery with an amateur sleuth, he or she has to have a reason to get involved that departs from what is good behavior and good sense in real life.

Loser wants to help the father of a little girl who reminds her of her daughter.

Caroline is suspected of killing her former best friend.

Many authors choose to have their protag or someone close to him as the suspect. This is certainly a driving force, as long as the rules of logic are applied. If you're accused of murder, are the people involved in the case likely to sit down with you and talk about it? And why would they be honest? It isn't like you can arrest them for perjury. If you testified in court that someone confessed to you, his lawyer would have you for lunch. "So you are a suspect in this case, but you want us to believe that Mr. X confessed to you that he did it. How convenient for you!"

Four vagrants witness a murder but can't report it to the police.

Another scenario is that the protagonist has special knowledge of the crime that he can't/won't share with the police, or he shares his info and they ignore it. If that were so, I suspect most of us would rationalize our way out of investigating on our own. "Well, yes, I did see Mr. X leaving the scene of the crime, but the police asked him about it, and he claims he was in Hoboken." End of story. Most of us don't have the time, the temperament, or the drive to chase down criminals. We'd grumble, "Nobody ever listens to me," and forget about it.

Princess Elizabeth wants to stop a killer.

Finally, there's the nosy protagonist who simply can't stay out of other people's business. I find these the hardest to relate to, and I've given up on books when the amateur sleuth's pushy behavior didn't make sense. The book I'm reading right now comes close, with a woman who travels all over England questioning people about a death she has no connection to. Her family keeps telling her not to do it, and she's been attacked several times now, but she's determined (dad-gum-it) and just won't stop. Luckily the author presents her as determined and stubborn, so her refusal to give up is at least tenable.

In defense of authors, we need a story, the story needs a protagonist, and the protagonist needs to keep going when the police either stop investigating or never really begin. My concern is with how well the author sets up the story. In the beginning, can I believe this person would take up the challenge of a murder investigation, and as we go onward, do I still like him or her because of it?