The positive link between attitude similarity and attraction is one of the fundamental outcomes in social psychology. However, attitude dissimilarity seems to be a stronger driver of this relationship than attitude similarity. The authors review the evidence on this similarity-dissimilarity asymmetry, and discuss two explanations. One is that people generally enter into interactions with optimism, and so supposedly neutral partners are often seen as mildly positive. Another is that dissimilar attitudes carry greater weight than similar attitudes in cognitive processes. Implications of these mechanisms for wider issues in person perception and attitude structure are discussed, connecting them with more recent theories of attitudinal ambivalence and evaluative space.