If I understand your question correctly, I'm assuming you mean that it's common for people to zero in on someone whose life isn't "all in order" and start "putting their two-cents in" (and worse) in that person's life and on his situation.
Why I think this can tend to happen lots of times is that human nature has a few traits that cause it:
1. When the person in question (the one who's down) is someone who has been on top and by whom others have often felt threatened, they can kind of like the fact that he's no longer on top. So they can kind of "live it up" and enjoy their chance to feel like the one who is on top.
2. When a person has never, or seldom, seemed to have his life in order; the trait in human nature is often a tendency to assume there's something inferior about that person or his judgment or willingness to do what he has to do. As a result, a very unappealing and often misguided belief that one is superior and knows better what the other person "should be doing" can kick in.
In fairness to some who actually mean well, there can be a parental instinct that people have towards someone who is down (even the person who is down is a perfectly sound, capable, wise, adult). With parental instinct comes an urge to tell someone how to do things, take control of the person's situation and/or contribute to the "down" individual's suffering consequences of choices the person with parental instinct didn't approve of.
3. Measuring other's choices/behaviors by our own: There's a tendency in a lot of people (certainly not all) to measure how "right" or "wrong" someone has been in his choices, or in his ways of doing things/thinking, by our own. If he's "more" than we are on one thing or another we may see him as "too much". If he's "less" one way or another than we are, we may see him as "not enough". In other words, if he isn't, and doesn't do, everything in just the same amount/way as we do, he's "not as ideal" according to our own measure of "ideal" (which in some people is themselves). So, people's thinking too highly of themselves is another reason they a) see someone as incapable of keeping his life in order, and b) incapable of getting it back in order. Then, in comes the attempts to offer "better ideas" and to step in and take control. :/