People are given the gift of free will, and that means the freedom to choose. If a person chooses to die because of their diagnosis, they are free to choose whether they should continue on, especially if their quality of life is severely diminished. By choosing to die, they can free themselves from the misery of what little life they have left.
Every suicidal person should not be allowed to legally commit suicide on a whim, they should instead be put in therapy and given help. However, there are many for whom death is a good option. Those with terminal illnesses or who have a very low quality of life due to medical issues should be given the option to legally die to end their pain and suffering.
Just as people can choose whether or not to have babies or modify their bodies, they should have the right to choose to die. People can refuse medical treatment that would prevent death, and they should be able to choose to hasten death as well. The state does not have the legal right to deprive people of choice about their own bodies or lives.
If a person is old or sick and dying, and he or she wishes to die and not receive medical attention, that is their choice and it's completely up to them. However, if a person wants a medical professional to assist with suicide, the medical professional should not be allowed to assist.