It's easier to see the good things in other people's lives than it is in our own. In our own lives, we see those good things as just things and nothing special. But when we see other people's good things, we see something amazing. I'm not sure why this is so, but I'm guessing that it's because we're more used to our own lives than we are to other people's.
Also we are more aware of our own lives, which includes the bad things, whereas we usually aren't able to see the bad in other people's lives. Therefore when we analyze the good in our lives, we also take in consideration the bad. But when we analyze the good in others' lives, we don't even think about the bad stuff they have as well since we are mostly unaware of them.
I also suppose that it is also human nature to be jealous of others, to desire more than we have and therefore lusting for the good things that other people have that we ourselves do not have. When we compare ourselves to others, we only focus on the positive points on the other side because those are what we want. Humans can only focus on one thing at a time and usually that thing would be in our best interests.
