I seem to ascribe emotions to a system -- more generally, I ascribe cognitive states, motives, and an internal mental life to a system -- when its behavior is too complicated for me to account for with models that don't include such things.

You mean like a psudorandom number generator?

Motives are easy to model. You just set what the system optimizes for. The part that's hard to model is creativity.

If I were somehow made smart enough to fully describe your behavior without recourse to what Dennett calls the intentional stance, I suspect I would start to experience your emotional behavior as "fake" somehow.

That's a bad sign. My emotional behavior wouldn't become fake due to your intelligence.