I agree 100 percent with zenbrat, at least - my observations about ISTp-ENFp interactions confirm your theory. May I ask if your interactions with ISTp-Tes match this description:

The relationship between Exxp-Ne and Ixxp-Te is one between two people empowered by specific achievements. Exxp-Ne has a habit of just doing it. Ixxp-Te has the habit of doing it really forcefully and emphatically so that its acknowledged for certain. There's a certain inclination for competition and attempting to subdue the other one.
Quote Originally Posted by mikemex
I have no idea which subtype is each; if subtypes exist at all. It seems to me that it has to do more with intelligence (not all ISTp are the same) level. Very intelligent ISTp are like very intelligent people of any other type: more interesting in doing something than just wasting time.
So, basically, the more intelligent you are, the less likely it is that you are interested in having fun? I really enjoy being stupid, then.