We're all a bit nosy sometimes, but there are some who pry into other people's affairs. The type who spy on other people, even if they aren't doing Nosey him/herself any harm or if they aren't breaking any laws. The type who busies him/herself with the private lives of strangers. Why do such people do it?
The simple answer is the lack of anything interesting or fulfilling in their own lives. There must be a lot of people who feel this way - there are a number of multi-national magazines - 'Hello' and so on, which pander to this mind-set.
I would think that it's not that much difference to the reasons that people watch soaps (or most TV, in fact). I imagine it makes you feel part of other people's lives, which as social animals, we are always trying to do, especially if we're lonely.
Fakey: I'm afraid I don't quite understand your hypothetical example, but I take it you are referring to good intentions. I really meant a case where the person gets involved, not with someone they care about, but with complete strangers. Incidentally, this question is meant to stimulate moral debate (it helps with my work), so thanks everyone for your answers. Sft: what do you mean by get a life?