For instance, in The Monitors Stuart is not just instantly attracted to Claire, he's convinced that she's the one person he wants to spend his life with. To persons not me, the question is 'why?'. To me the answer was obvious: Stuart's different to most of the people he meets -- 'the normals' -- but Claire isn't a normal. She isn't like him either, but they've got that point of reference in common on top of the mutual attraction, and the shared sense of humour that enables them to overcome their initial communication difficulties. Of course it would have been easier for other people if I'd put all that in a first draft without being prompted.
As mentioned in discussions with firynze earlier, having an issue pointed out in one story can make one more aware of similar issues in other stories, and character motivation is definitely one of those areas where I sometimes need to be prompted for clarification.
As a hypothetical example, a character might run through the important information she needs to impart to her co-worker, but not want to say any of it out loud there and then because they're busy saving the world/robbing a bank/painting the Forth Bridge and interrupting their task right then to discuss matters not directly related to it may result in failure of their current task. But again, the reader needs to know why this discussion has to be left until later, no matter how important it feels to the viewpoint character.
Or if a character is covering for someone else's crime/mistake/failing, we need to know what is so important about that other person that our hero is taking the blame. If it's a family member, would they do the same for any relative, or just certain ones? If it's a friend, why is that friend particularly important and deserving of this possible sacrifice?
In that second example, showing is possibly more important than telling. We need to see how far the character is prepared to go for her family and friends under other circumstances in order for the big altruistic action to feel right. Or we need to see all the times she doesn't put someone else first, and then see why she makes an exception in this instance.
And then there's the difficult case of the character whose motivation is at times, and of necessity, uncertain. The character who mainly just wants to survive from one moment to the next, and has had no real opportunity for long term planning. I'm struggling slightly with this one at the moment. Later in the story, she gains a motivation to keep going and to make long-term plans, but where I'm at now in my editing process she's still trying to stay alive and keep herself safe from the bad guys who may be out there looking for her. Each decision she makes or puts off making has been based entirely on what would be the worst case scenario vs would anything worse happen if she did nothing. It's not the easiest scenario in which to convey tension, but hopefully some smoothing out of the rough patches of writing, and some added background details will make all the difference for reader engagement.
[ETA:] It strikes me that another character type I love may be even harder to write motivations for: the character who has successfully escaped their former occupation and just wants to be left alone, in spite of the rest of the cast's continued efforts to draw them back in.
How about you? What are effective and not so effective ways in which you've seen character motivation portrayed? Under what circumstances do you find it difficult to convey why a character is acting in the way that they are?