If a married person passes away without an estate plan, then by law his or her assets will go to the surviving spouse, which is what most married people would want.
However, as the Fond du Lac Reporter points out in "Easing the burden: Estate planning for singles," things are a lot more complicated for single people.
If a single person passes away without an estate plan, then there is a statutory scheme that determines who gets the deceased's possessions. However, it will depend on which family members are still living and assets may not go to the people the single person would have wanted to have them.
For example, all of a single person's assets might go to their parents when the deceased might have preferred to give them to a niece or nephew. Similarly, if a single person becomes incapacitated, then no one by default has the ability to handle his or her financial or medical affairs. It will be up to a court to determine who gets that responsibility.
For those reasons it is important that single people do not think estate planning is only something for married people. Single people might need estate plans even more then married people.
Reference: Fond du Lac Reporter (June 24, 2016) "Easing the burden: Estate planning for singles"