Marriage is WORK, period. No amount of dating, living together, number of people you've dated or lived with, nor how old you are can prepare you for just how MUCH work marriage is. Add in kids and well, you start thinking about locking yourself in the garage with the car running. j/k
You get married b/c you really love this person and you don't want to die alone. A few years later and you fantasize about how nice it would be to die alone. In the quiet. Surrounded by the things you love. Like your car or your music or musical instruments. Or your comic book collection. Or watching sports, going bowling or hiking or skiing or JUST DOING NOTHING. You know, that life and hobbies you USED to have before you got married. /sigh