why can't people just be happy with what they have?

I don't understand it. Why not be happy with what you already have? Why the constant search for more and more that only leads you straight to total disappointment? I don't get it. Why do people feel the need to put themselves through emotional and physical pain over and over again? I guess some people just can't be happy. I mean I wish we had more kids, I wish the kiddo had a sibling. For awhile I was ready to deal with the fertility doctor, but he was an ass and now I'm fine with it. We have a small family and maybe one day we'll add to it by adoption or by the birth of another child. IF that day comes, I will welcome it, but if it doesn't, I will not mourn the loss of what could be. Yeah, I'd love for the hubby to be higher ranking, but again, if that day never comes, it's ok. We can manage with what we have right now and still live a pretty complete life. We're not struggling by any means. I'm happy with things just the way they are. Why can't other people see how ridiculously full their own lives are and be happy with it?