We've talked about this before in terms of cozies, but never MTS at large, that I know of.

What's your character's motive for what s/he does? Whether it's committing a crime or his/her way of going about solving a crime, is it clear why s/he does what s/he does?

I recently had a painful rejection that said I just don't understand why (character name) would (do what he did). And I thought, well, because he's crazy.

That's enough motive, but it has to be clear to the reader, and obviously I fell down in that department.

What's a good motive? (This goes beyond TSTL, by the way, which is a problem all its own.) Obviously "I've been framed for murder and need to clear my name" is a pretty kick-ass motive. Is "This guy is so in love/lust that he misses some pretty big clues about her character" a good explanation, a good enough motive for him overlooking her as a potential suspect? (I know--it depends. )

I'm not sure what I'm looking for, exactly, except whatever you feel like putting out there.