My take on it is totally based on a narrow scope of experiences, so I could just be talking out of my ass. But my roommate is an INTP, and based on my interactions with him over the past few years, I think a big part of it is curiosity. I think INTPs connect with us initially because they see how observant we are, and they appreciate how thoughtful we are. But I think they stick around because while they see this, they're curious about how we seem to understand people effortlessly (to some degree). They see that we know something, but the fact that we weren't led to it by a trail of logic intrigues them. I think they genuinely want to know how we work and make decisions, because while we seem so much like them, they struggle to make sense of our Fi function.

His girlfriend and I are friends, too (she's also an INFP), so we all hang out a lot. We know that things get frustrating when he demands an explanation for something, but we can't give him one that satisfies. Of course, it was incredibly irritating initially to always be asked to explain myself, but at least in his case, his want to listen and make sense of things gives us an outlet to talk freely about things, which is huge when you spend most of the day probably being quiet. Even though my interests and motivation don't always line up with his, he seems to always want to know more about them. It gives me practice in articulating the ideas in my head, and it keeps me thinking about whether I'm making sense.

Given that, I'd say there's no reason that an INTP/INFP relationship would inherently not work out, but there is inherent dissonance. I think it really requires compromise in that the INTP be more understanding of the INFP's Fi-based decisions, and that the INFP be patient with the INTP's need for things to make logical sense. I'm sure that if the right INTP girl came along, I'd love to give it a shot.