Due to the rise of affective polarization, Americans today not only feel psychologically distant from out-partisans but also prefer to build political and non-political social relationships with co-partisans. However, it is not clear why we see such patterns. Is it because people consider co-partisans to share similar demographic and socioeconomic backgrounds and thus feel more comfortable interacting with them? Or is it because they believe that co-partisans share their policy positions? To answer this research question, this paper conducted original survey experiments that were suitably designed to study causal mechanisms. The analysis of the experimental data revealed that both group traits and policy issues play important roles in explaining the effect of partisanship on both political and non-political relations. We also compared the explanatory power of group characteristics and issue positions to demonstrate the latter more strongly drive the effect of partisanship.