"Am I happy? What is happiness?" are questions that seem to baffle my ISTP. Intuition is way, way down on the ISTP's function of hierarchies. It is really difficult for him to tap into it at all, much less while feeling time pressure.
"You will never be happy if you continue to search for what happiness consists of. You will never live if you are looking for the meaning of life."
I tend to hold onto this view. When I start dissecting what happiness is...well, I don't really do that. But if I did I'm sure I'd go into the "DLLD" zone. I don't feel like it needs to be defined and if someone needed this from me it would be a bit stressful.
You seem to have a good understanding of your ISTP. I've noticed this with ENFP's. Your genuine concern and curiosity about people allow you to get close to people like us. Not easy Makes for a good partnership. Good luck!