Yeah it's the reason I don't think I'd be able to permanently own an animal. I watched my dog die a few years ago, who I had since I was 7, and it was one of the worst things I think I have ever witnessed. It really makes me contemplate deeply the merciless reality of our universe...
I worked at an animal rescue for two years when I was in college, and we rescued this cat who had been hit by a car. Somehow he managed to survive and after several surgeries and rehabilitation he recovered more or less completely, aside from having a permanent limp. He was so terrified of everyone though, and hid in a corner underneath a chair whenever anyone came in his room. So I laid on the floor every time I worked there, and slowly he came out and wandered freely, and after a while he would come up to me and eventually let me pet him. He got so much better that he was ready to be adopted, but a few days before his new family was going to come and pick him up, he contracted a virus that is fatal to cats. The family adopted him anyway even though we had to break the news to them, and they vowed to care for him in the final days of his life. They called us about a week later to tell us he had passed away.
There were other instances where we had rescued very young (weeks old) kittens who tested positive for feline leukemia and were put down. It's really a big reason why I can't be involved in the actual rescuing process anymore, because it's just way too distressing to me. I love animals, they are innocent creatures, and it's so unjust in my opinion that in way too many cases, more animals are born because people don't take care of their pets, and then they suffer and die. In many cases I could say similar things about people, but I won't go there in this thread.
That being said, the volunteer work I've done with animals in more recent years has been a bit more removed, like helping to clean the adoption center and donating things, but I do miss being around animals, so I feel like fostering would be perfect.