I don't think there's a reason for you to feel dumb: there are a lot of candidates that are the opposite of the grandparent, and want to do "it" for the rest of their lives.

However, a lot of times the "want to do it forever" part could be a sign that the candidates doesn't want to learn anything new, which is indeed a very bad sign.

On the other hand there are people who want to want to keep doing tasks so they get a deeper understanding of the domain, as mentioned elsewhere in the comments... and you can't afford to say no to those people.