It would make sense for kissing to be favorable for evolution if it led to increased arousal and consequently a greater inclination to have sex. When a (heterosexual) couple is kissing, testosterone can pass from the man’s mouth to the woman’s, which may make her more receptive to sex (and the passing on of their genes). But it turns out that though people certainly kiss when they’re aroused, there’s not much evidence to suggest that it works the other way around, where kissing causes arousal. And though some other species such as bonobo monkeys also kiss, it doesn’t have quite the same association with sex as it does for other humans. This suggests that there are other factors at play than just the initiation of play time.