I believe life is monumentally precious and have respect for all life. I wouldn't kill a spider and I don't think we have the right to take life, even when we call it being kind.
Life is the most magical thing in the universe and I'm in awe of it daily, from tiny bees and spiders, to sea life, mammals and humans.
However even I have exceptions. Pieces of shit that murder, abuse and torture don't deserve life because they clearly don't respect it or have any regard for the happiness of other living beings.
In disease like Alzheimer's, cancer, paralysis etc., I still believe it's not our place to end that life and we should let nature take its course. I know it might seem cold and uncaring but it's not I assure you, my heart breaks for the suffering of others, but once you end that life it's gone, over, there's no coming back and there's no hope. What if someone with Alzheimer's ends their life then there's a cure found one month later?
Also as long as someone's alive they can feel love, joy, experience this beautiful world, but when it's over there's nothing.
However, I wouldn't stop someone who decided on euthanasia. I'd tell them how I feel (if asked or if they were a loved one), but the decision is ultimately theirs.
Regarding pain and suffering, as pessimistic as it sounds we all experience pain and suffering to different degrees at different times, that's as much a part of life as laughing, joy, loving, being loved. Yes some experience much more than others and that's tragic, but at what point do we say that the level of pain and suffering justifies ending or taking a life?