Argumentative discussions are commonly described in adversarial terms, but it is not obvious that argumentation is necessarily adversarial. Here I argue that adversariality is not an essential element in argumentation and in fact it must be avoided because it discourages reasonable behaviour. Reasonableness, characterised here as willingness to modify one's own beliefs in the light of good arguments, is jeopardised by adversarial goals. That gives us a reason to reject adversariality in argumentation, since reasonableness contributes to some of the greatest goods that argumentation can provide.