People always seem to want what they don't have and life with or without a partner is often foremost on their list of what they want in their life. Single people want someone to share their life and help them get through it, but they often forget the downside of someone expecting their help as well. Married people have someone like that, but they crave the freedom and lack of responsibility that belongs only in the realm of singles.
There will always be people who are content with whatever lifestyle they are living. Some people are loners who have no intention of ever finding a spouse, and they are happy to make all the decisions in their life. There are married people who cannot imagine life without their spouse, and they are intensely grateful to have found their perfect match. These people are few and far between, and there are always those who wish they had taken a different course in life if only for a short time.