Curiosity is one of the basic traits of mankind. And i believe it is from this curiosity, that we get a tendency to bother about what other's are doing.
As an onlooker, we might learn a lot about the people involved in real situations but the fact is that we don't realise even half of what they are going through. It is more like watching a movie where you know whatever happens on the screen will never effect you in any way.
Like it is easy to predict the sequences in a movie and and say OK this is a tragedy movie if it has a sad ending or say "everyone lived happily ever after" and exit from the theatre. Similarly as an onlooker we might exit from the lives of the people involved after certain episodes because we lost touch or because their part in our lives are over. Then all that we know is they had such a situation in their life which they handled in this particular way and the result was so and so.
so all that is left for us in that is probably a lesson of how we should or should not handle a situation if have a similar one in our life. But the truth is once we face a real situation, that might not be the case. It is only then we realise why people in that situation earlier took those decisions. Most of the decisions we thought were foolish or stupid as an onlooker will be the most probable ones when we are really in to it.
This difference is occurring mainly owing to our emotional instabilities or rather loss of our rational thinking when we are having emotions attached to a problem. As an onlooker we rarely have emotional attachment to a problem, so our thinking is more rational and based on facts which are not hidden by silly emotions.
So the best way to handle a problem or to find the best solution is to think like an onlooker or rather fell like an onlooker and being a critic of oneself. This is possible only if we are able to detach ourselves from the real emotional person and view it from the onlooker's angle.