My coworker and I have discussed this many times, and it is a rational reason in my opinion. I still feel it is worth the joy in having the dog to suffer the pain of losing it, but I know my coworker is to the point where she feels she cannot go through the loss again.
My feeling is that death is a part of life (the worst part), but you cannot stop living in order to avoid it. In my case I feel life will be more full having pets. Our English Bulldog just died in December, and I am overall happy to have been able to share in such a special, though short, life.