I have had several loved ones end their own lives, for selfish and non selfish reasons. It ends their pain, but causes so much more pain, exponentially in others. It isn't worth it.
The fact that you have the right to do something doesn't mean you're right to do it.
I will disagree with this on one respect. People who are terminally ill who choose to die on their own terms deserve respect. I'd rather see a loved one end their own life with dignity rather than die suffering with their dignity and humanity completely erased by suffering. If you've ever seen someone die over the course of months or years with absolutely no joy in life and nothing but suffering, you know what I mean.
Cancer, Alzheimer's, whatever, there are some terminal illnesses that rob you of your humanity, your personality, and your soul, and your existence has no benefit to you or anyone else.