Hm.. I dated an INTP for 3+ years, and we're still pretty close. We're a -lot- alike, but he's definitely a lot more into theoretical things and always hungry for intellectual conversation. He's absolutely brilliant. Definitely has more intellectual depth (at least when it comes to science and money.. haha) than most 23-year-olds I've met. I'm not usually interested in the same things he is (I love science and math, but get bored with theory, which is what he likes), but I know he needs someone to listen to him, so I always do my best. He actually is insecure (NOT saying all INTPs are), but mostly because I think he feels like he can't relate to anyone. He can't find anyone to talk about all his interests with him, and thus feels lonely a lot. He often voices that he doesn't know what he wants out of life, or that he simply can't find his calling. So maybe if you met an INTP that -was- insecure, this may be a pretty legit reason?
Maybe I should get him on here to talk to all you other INTPs Oh, and if you other INTPs have any advice for him, I'll be happy to relay it... :P