Friendship (philia) is the central focus of the arguments in Lysis, and love (eros) defines the frame in which these arguments take place. The dialogue begins with Socrates offering to help Hippothales figure out how best to court the boy Lysis, with whom Hippothales is hopelessly in love. Socrates's main objective in this regard is to show Hippothales how to humble Lysis into desiring him as a teacher, rather than by inflating Lysis's ego with praise and thus making him harder to get. None of this can be understood without some basic knowledge about how love relationships between men and boys functioned in Socrates's and Plato's Athens. The key to understanding how these relationships operated lies in the blending of passionate, occasionally physical love with the inspiration given by the ideal beauty of the youth and with the role of the older lover as a teacher of manly ideals and wisdom (see Lover in the terms list for more information). This is the framework for Socrates's conversation with the boys, as Hippothales watches the demonstration from behind a nearby pillar.

Friendship, such as that enjoyed between Lysis and Menexenus, is the topic addressed within this context. Socrates considers a number of hypotheses about friendship, shifting a bit from an attempt to describe what person (or role) is the true friend to trying to find the universal cause of desire. Briefly, the possibilities for defining "what is the friend" include: the lover, the beloved, the like, the unlike, and the good. All are rejected, primarily due to the seemingly intractable problem that like has no reason to befriend like.

By the end of the dialogue, a rough model for further investigation seems to have been set up: a true account of friendship has to explain why two people need or desire each other, but cannot allow for "monstrous" possibilities such as the good befriending the bad (or the just the unjust). The trick is to explain desire without allowing it to encompass bizarre or unacceptable situations. In fact, this is precisely the situation with regard to Socrates's attempt to show Hippothales the right way to put his desire for Lysis into effect. Thus, friendship and desire end up sharing a common ground in questions of desire.