Looking for misery...

My oldest daughter causes me some confusion lately.  She is 8 years old and loves to be miserable. I just dont understand it. She had the perfect opportunity today to be spoiled without the annoyance of younger siblings. She had my undivided attention and she pouted and whined the whole time. Im annoyed and aggrevated. I dont understand why she just cant be happy with what she has. Example: we went sledding, great big hill, very fast and steep. Perfect!! She whined that the hill across the way was bigger and better and she wanted to go to that one. So we said ok but its not better. You will regret the choice but we arent coming back to this one. So we drove over and she immediately said it is smaller. She went down twice and then sat and pouted. So I asked if ahe was sledding or leaving. She choose leaving. Now she moping and pouting. How do I teach her to be happy with what she has? And to realize how blessed she is to have what she does? She always wants what everyone else has even if its not better than hers. I dont understand.  What have I done wrong?  Ive tried to teach my children these lessons but apparently ive failed. :(