"Every silver lining's got a touch of gray"
Even good things can have a tinge of regret to them. Consider: Mary is a popular girl. Both John and Jim have proposed. She says yes to John and everything's fine. She's happy mostly, but sometimes she wonders what life might have been like with Jim. Well, here's the answer. SOME things would have been better, but most things would not have been. She made the better choice. Still, there are those days when she feels restless. There it is, that touch of gray.
This notion of a touch of gray in an otherwise good situation could be applied to a job, to children, to one's location, to almost anything. Next time you come across that famous poem about two roads diverging in a wood, remember that--unlike what we're often told--there usually is no "right" path, just one that's better than the other. They are just different, and despite the charms of one road, there will always be a wistful feeling for the one that wasn't taken. Choose differently and the wistfulness only increases.
Your challenge is to write about something that is basically good, or satisfying, or positive, but that nonetheless contains some element of gray. To paraphrase the Monty Python sketch, where is the ambiguity? In your poems, dear poets, in your poems.