At the prospect of parting:
End of an affair.
Have you ever been in a relationship that's deteriorated so far that you barely even talk to each other any more, where you realise it's 'over' and are just awkwardly waiting for one of you to summon up the courage to say so? And then, of course, just as you've got used to the idea and are feeling ready to move on, the damn girl starts dressing really sexily again... and it taunts you with memories of how things used to be, stirs up a flurry of doubts and regrets.
Beijing always does this to me. Just when I think I've really had enough, when I just can't take the craziness - the stupidity, the dishonesty, the selfishness, the homicidal behaviour on the roads, the crowds, the pollution, the corruption - can't take any of it any more, when I'm planning to leave and looking forward to it and thinking I probably might never come back... then we get a spell of idyllic weather, and a lull in work in which to enjoy it, and... the pangs of uncertainty start to tear at the heart. We had some good times together, didn't we?