Jury Nullification arises from the constitution's guarantee of the right to a jury trial. This guarantee creates one of a few constitutional obligations on the individual: The obligation to judge your peer, as a layperson.
The judge has a slightly different duty, due to "Separation of Powers". The judge is charged with enforcing legislated law. The judge is not permitted to evaluate whether that law should or should not exist; the judge must presume that the law is valid unless it conflicts with a superior law, such as the constitution. This is why the judge must treat nullification as a secret: They violate the separation of powers as soon as they tell the jury they are free to ignore legislated law.
You, the juror, are not subject to the separation of powers. As a member of "We The People", the Constitution derives its powers from you, not the other way around. Your duty, as a juror, is to provide the accused with their right to be judged by a jury of their peers.
Your obligation arises from the Constitution, and to the accused. Where legislated law conflicts with the Constitution, the Constitution supersedes the legislated law. Where you, as a layperson, believes that the legislated law does not adequately address the circumstances of the accused, you are not just "allowed" to find the accused not guilty; you are morally obligated to do so.
You will be asked if you hold any beliefs that would prevent you from rendering a judgement solely on the basis of the law, which would make you ineligible to serve on a jury. The constitution is law. My beliefs arise from the constitution, and I hold no beliefs that would prevent me from rendering a judgment on the basis of anything other than the law. I can honestly answer "No".
If jurors can ultimately do whatever they want, what stops them from using nullification all the time?
We don't actually know how often juries nullify. It is impossible to distinguish between an acquittal on the basis of "Reasonable Doubt" and an acquittal on the basis of "This law did not envision this defendant's specific circumstances".