As discussed in the entry on "Likeness and Identity" above, desire undergoes some very strict analysis in this dialogue. Most notably, desire is judged to depend on difference rather than on identity. This, however, is precisely what frustrates all of Socrates's attempts to account for desire, since a desire defined only by difference ("unlikeness") would lead to "monstrous" friendships, like the good befriending the evil. Nonetheless, it is striking that all of the twists, turns, and rejections of Lysis culminate in the one final theory that friendship is simply due to desire. Bound up in this final assertion is the intriguing but underdeveloped quality of the "congenial," which appears to be an attempt to theorize how two things can be different in a harmonious way (this would solve the problem of "monstrously" inharmonious friendships). Unfortunately, the congenial is quickly judged to be little different from the like, and to have the same problems. One other property of desire is asserted in the dialogue: desire, in itself, is neutral, like hunger. It is this property of desire that allows Socrates to throw out evil as the thing that drives people toward the good of friendship; since desire is neutral, it would be around even if all evil disappeared.

Lysis is notable for the ways in which it weaves erotic desire and friendship into a complex tapestry. Throughout much of the dialogue, the causes of both eros (passionate love) and philia (fondness, friendship) seem to overlap quite a bit precisely on the issue of desire. Desire also binds the context of the dialogue (Hippothales's erotic love for Lysis) to its content (Socrates's discussion of friendship).