I imagine, considering the nature of human beings, it's difficult for a type that is so significantly different from other types to find the same level of happiness considering happiness in the brain hinges so much upon human interaction, something that doesn't come as easily to an INTP because of the aforementioned reason among others. The group (society) simply doesn't have the INTP's best interests in mind leaving them with few avenues to fulfill their unique desires and aspirations. Now it is certainly up to every person how they want to act and interact in society. One could suggest that INTPs conform more to the rest of the bannana bunch and derive happiness that way, but we *should* all know it's not so easy as that. Above all else INTPs have a respect for individuality and their own freedom. Often times that means severing the bonds and constraints of society and its norms in order to be who they want to, and should, be.
Everything comes at a price though, and the freedom and might of individuality that an INTP may seek inevitably means they will promote the further separation from others. Being psychologically strong for an INTP is being able to be comfortable alone, to be able to entertain yourself, and to occupy yourself with things that give you a sense of fulfillment without dependence on others for this stimulus. It is a way to circumvent the fundamental gap that exists between what the INTP wants to be and the demands human nature puts upon us to conform to the group and depend on interactions with other people as if it were a form of sustenance in itself.