as long as they are mentally competent.

I believe it to be a Quality of Life issue.

My close friend died recently of complications of AIDS. Or so we thought. We all thought his long battle had finally worn him down, and he chose to forego dialysis and just take a morphine drip. He had been a game fighter, and a valiant and courageous inspiration to all of us in our circle.
It wasn't until after his memorial service we were all told his decision to terminate his suffering came about because he had also been diagnosed with pancreatic cancer, and the pain had completely overwhelmed his ability to cope. He fought like Hell to live until his quality of life had so deteriorated that it was no longer worth the effort.

Sometimes, its better to let them go a day sooner, rather than allow them to endure the suffering any longer. I hope when my time comes, my friends and family will allow me that same dignity of choice.

a society would prohibit this on religious grounds.