R. Yishmael says: Come and see what hatred causes. It leads to slander (viz. Ibid. 14, "And he libel her and give out about her an evil name.")
From here it is derived: If a man transgresses a "light" mitzvah, he is destined to transgress a "grave" mitzvah. If he transgresses (Vayikra 19:18) "and you shall love your neighbor as yourself," he is destined to transgress (Ibid.) "You shall not take revenge … and you shall not bear a grudge," and (Ibid. 17) "You shall not hate your brother" and (Ibid. 25:36) "And your brother shall live with you" — until he comes to the spilling of blood. This is the intent of "If a man take a wife, etc."