Often, people who don't have spouses or children don't see the need to have an estate plan. However, for many reasons, it can be even more crucial for single people to have a plan in place than for those who are married (although it's important for everyone).
What happens to your brokerage accounts after your death? Do they just pass over to whoever you have named as your beneficiary? Like any of your other assets, you should do some advance planning for assets you hold in the market. Your brokerage account assets will be transferred to someone, and there is a process they go through.