In the Netherlands, health care is part of its social security system. Therefore, health care insurance is mandatory for all residents in the Netherlands that are subject to Dutch social security law.
Am I subject to the social security law?
When you or your partner work in the Netherlands and pay taxes, you are subject to the social security law. This might differ when you work temporarily in the Netherlands, or when you are student or PHD candidate. Check the website of the Sociale Verzekeringsbank if you believe you might be exempted from the healthcare insurance duty.