In my opinion the motivation for the main character to behave in a certain way was not reasonable. As the book continued and the protagonist carried on using this rather flimsy reason for their increasingly strange behaviour I lost my belief, my sympathy and my patience.
I did finish the book, instead of throwing it out the window, because it was in part a mystery and I wanted to know what happened. But I was not happy.
I felt let down.
It seemed to me that the author needed a reason for their protagonist to behave in a fairly stupid, dangerous way in order to furnish the plot but the motivation she'd settled on was not good enough. No one would behave in such a way for such a reason. It was ludicrous. Ludicrous!
While grumbling under my breath the writer part of my brain switched on and I started thinking about my own work. Were my characters properly motivated? Or was I simply pushing them into certain behaviour I needed for my plot?
Luckily at this point I had some input from an editor who flagged up a particular character in my m.s. I looked up my notes and saw that I had written nothing about this person despite their relative importance to the story. Instead I'd simply slipped them into a standard role without even knowing in my own head who they really were and why they would behave like that. This was bad.
I went away and thought about it. About all my characters in fact and I started imagining little cut scenes for them from their past. They became very quickly more well rounded because I understood them more. I worked out their relationships with each other, the dramas that had shaped them. Of course I don't need to put all this backstory into my m.s but I need to know it because plot comes from character.
I now feel that my story will become stronger and richer with this added knowledge, the characters will live and breathe and behave the way they're supposed to, NOT just because it's the way I want them to. This also resulted in a few eureka moments, created by my new understanding, so much so that I'm quite excited about sitting down to my next draft.
Of course then I started wondering why I hadn't just done the character analysis before I started writing, so that all that deeper understanding was already present in my first draft. But on reflection I think the understanding in many ways COMES from writing the first and second drafts.
Anyway, it was a useful lesson to me. Readers will not accept flimsy reasons. They need to feel the motivation for behaviour is reasonable, compelling even. Otherwise they can get quite cross and lose belief in your whole book and that as I have seen first hand is not the response we want.