My mother had Multiple Sclerosis and my father did all her care. She was mostly immobile. She could sit in a wheelchair for an hour without it wiping her out for the rest of the day, but going out to meals left her exhausted. He lifted her in and out of bed, in and out of the wheelchair. Luckily his health was/is excellent, and he wouldn't have had it any other way. He did have home help, a woman who came in and did laundry and would wash my mom's hair when she came in, but he did basically everything. I understand that for some that is too much. My mom passed two years ago and my father is remarried to a younger woman (he is in his 70s and I think she is just under 70, LOL). He's going a bit crazy with travel and doing fun stuff. He had to forego a lot of that for a long time and he deserves this. Most people would look at my dad and think he's in his 50's. Hopefully I got more of his genes for remaining healthy into my older years.
It probably has something to do with the illness the person has. While my mom wasn't all there mentally at the end, she got a bit confused at times, for most of the years he cared for her she was completely lucid, just unable to do most things. I can imagine caring for someone with Alzheimer's could be a lot tougher, especially emotionally.
