Filling in the Holes

Writing fiction can be a real pain in the butt. I have no problem thinking of interesting overall plots, I've got about a dozen ideas I've written down that are just itching to be brought to completion by way of the story, what's difficult is filling in the empty spaces.
For example, say I have an idea where a character named Fred is suddenly distraught over something and searches out his friend Stan for help. Maybe this is a pivotal part of the plot, maybe Stan learns something about himself through helping Fred, or maybe this is the turning point for Stan and he rejects Fred as a friend and becomes a psychopath (Stephen King style, of course). In this example we know who is involved, we know the initial situation between the two (they are most likely good friends if they come to each other for help with such matters) and we know what we want the desired end result is. What we do not know is something that seemed completely insignificant until it came up in the middle of writing the scene: what is Fred distraught over?

So obviously as a writer I try to explore the simplest and most realistic options available; work, school, money, or some personal romantic relationship. But how can I make it work? How does (A) combined with (B) equal (C)?

There is a story I was working on all throughout high school in which I encountered this very problem. It was about how a particular man dealt with his girlfriend of several years breaking up with him just when things seemed to be going best. What I realized halfway through the story was that I knew she had to be the one to change her mind and want him back, but also needed a very good reason for breaking up with him in the first place. Seems like an impossible situation right? It's like trying to figure out an elaborate reason that a person is murdered but the murderer is regretful later on that's still simple enough to understand, and "misunderstanding" is a cliched and, quite frankly, bologna reason. Luckily in that instance I had a friend go through a similar event, in fact it was scary just how similar the situation was; the only difference was the amount of time they had been dating. She broke it off and then took him back a week later. And throughout the whole week they were apart he and I spent time trying to figure out what happened, what he did wrong or what changed her mind. Once she came back to him he informed me of his uncle's theory:
"She was scared of how she felt"
And there it was! It didn't matter if this was true for my friend's particular situation, but I had just been hand-served a perfectly fitting piece of the puzzle! What if she loves him so much that she's scared of losing him, and reacts offensively before she fears that he does? BRILLIANT!

Unfortunately the reason I blog about this now is because I have run into just that same situation. Life is complicated and figuring out reasons for things happening is difficult when you can't just ask a living person involved in it to explain it for you. I have an idea or two floating around in my head, but I just don't think they fit just as perfectly as they need to. I've texted four creative minds I know for suggestions, but I am restrained from asking the people who matter most because to get their advice is to give away part of the plot, and these are the people I want to experience the story as it is meant to be experienced: lifelike and with no hints to the future's events.
If none of my four creative minds report back with anything of use, I'm on my own.

This is one of the thrills of being a writer.
-Ryan