So many reasons...
1) its hilarious and punny.
2) it brings a tear to the eye.
3) the fact that its based on the aftermath of a third world war - making it realistic.
4) its wild fantasy and craziness.
5) the deep personalities of each of the characters and how they face their issues.
6) the use of music makes you think, and can give you something to remember the show by.
7) the beauty of the shows setting - the land of Ooo.
cool how themes are brought into it relating to real-life problems - father/daughter problems, relationship problems, etc.
9) the characters can become your idols, like you want to be like them, act like them.
10) its different to other cartoons; it makes you want to believe it the realness of it all, believe in the magic, and be part of it, like its the future.