Personal relationships, particularly romantic ones, can be the INTJ's Achilles heel. While they are capable of caring deeply for others (usually a select few), and are willing to spend a great deal of time and effort on a relationship, the knowledge and self-confidence that make them so successful in other areas can suddenly abandon or mislead them in interpersonal situations.
This happens in part because many INTJs do not readily grasp the social rituals; for instance, they tend to have little patience and less understanding of such things as small talk and flirtation (which most types consider half the fun of a relationship). To complicate matters, INTJs are usually extremely private people, and can often be naturally impassive as well, which makes them easy to misread and misunderstand. Perhaps the most fundamental problem, however, is that INTJs really want people to make sense. :-)
This sometimes results in a peculiar naivete', paralleling that of many Fs -- only instead of expecting inexhaustible affection and empathy from a romantic relationship, the INTJ will expect inexhaustible reasonability and directness.
Probably the strongest INTJ assets in the interpersonal area are their intuitive abilities and their willingness to "work at" a relationship. Although as Ts they do not always have the kind of natural empathy that many Fs do, the Intuitive function can often act as a good substitute by synthesizing the probable meanings behind such things as tone of voice, turn of phrase, and facial expression. This ability can then be honed and directed by consistent, repeated efforts to understand and support those they care about, and those relationships which ultimately do become established with an INTJ tend to be characterized by their robustness, stability, and good communications.