http://www.antipsychiatry.org/suicide.htm
One reason some oppose the right to commit suicide is theological belief that is sometimes expressed this way: “God gave you life, and only God has the right to take life from you.” Using this reasoning to justify interfering with a person’s right to commit suicide is imposing religious beliefs on people who may not share those beliefs. In America where we supposedly have freedom of (and from) religion, this is wrong.
Another reason some people believe it is ethical to interfere with a person’s right to think about or commit suicide is belief in mental illness. But a so-called diagnosis of “mental illness” is a value judgment about a person’s thinking or behavior, not a diagnosis of bona-fide brain disease.
So-called mental illness does not deprive people of free will, but on the contrary is an expression of free will (which reaps the disapproval of others). Those who say mental illness destroys “meaningful” free will or who call the beliefs of others irrational (and therefore necessarily caused by mental illness) are accepting the idea of mental illness as brain disease without adequate evidence or are refusing to accept the beliefs of others only because they differ from their own.
Sometimes people oppose the right to commit suicide because of belief in a sort of entirely non-biological mental illness. The error of this way of thinking is that without a biological abnormality the only possible defining characteristic of mental illness is disapproval of some aspect of a person’s mentality or thinking. But in a free society, it shouldn’t matter if the thinking of a person meets with the disapproval of others, provided the person’s actions do not violate the rights of others.