I'm not sure how to word this but I hope I get the question across clear enough.
I'm with an ISTP and I notice that the source of all his excitement / energy / lust for life comes from either: inanimate objects, socializing with people he doesn't know too well (and there's a strict criteria for that, below) & pets.
With people, he only acts truly 'stimulated' if there will be people among the group that he is acquainted with but, under those conditions, the more strangers there are, the more excited he gets. (Example: he would never be dragged to a party where he didn't know anyone well enough but he still actively seeks out situations where he's surrounded by new faces.)
With anyone he knows well, he tends to lose his energy. Even if the event is exciting or he seems to be enjoying himself, his mood never goes any higher than "comfortable" (unless he's really drunk). But if you offer to buy him snacks, a gadget or show him a cat, he's near extroverted about his excitement.
It seems fairly normal, I suppose, to a point. I have never seen him stray from this pattern though. I'm pretty sure I never really saw him excited or thrilled over anything we did together (unless many others were involved) once I knew I had his trust. I had an idea that maybe he just naturally likes things that don't expect anything from him... Like the affection he can display towards pets is, just wow.
Any other ISTPs or SPs feel they relate to this? I want to understand it better. Also, any ISTPs ever actually been really excited about a specific person? And I don't mean an activity with a person or a chance to get away from boredom but really anxious/happy to see or be around someone?