I think maintaining interest is all about what you expect and want from diving. My wife and I certified together. For me, first it is about having a hobby with her that was ours together. Second, it is about going and seeing new things and exploring. With a basic cert, I would probably grow tired of the second reason after many dives. I'm working on expending skills and environments to keep my second reason as strong as the first.
If diving grows old, consider varying experiences before throwing in the towel. Cold water, ice, tropical, fresh water, salt, rivers, mountain lakes or mud pits - all have something a little different. Don't let it grow old because of not trying more than one option.