At this [Hippothales] blushed; and I said to him, O Hippothales, thou son of Hieronymus! do not say that you are, or that you are not, in love; the confession is too late; for I see not only that you are in love, but that you are already far gone in your love. Simple and foolish as I am, the gods have given me the power of understanding these sorts of affections.
This passage has very little hard philosophical content, but it is precisely for that reason that it occupies a crucial place at the beginning of the Lysis. Socrates has just asked Hippothales who his "favorite" is and the exchange of blushes, boasts, and jests that follows this inquiry does the initial work of setting the frame for the entire dialogue. Two things in particular should be pointed out. First, by asking for Hippothales's "favorite," Socrates is asking neither about his best buddy nor about his female crush. The "favorite" is a young boy in whom Hippothales has mingled his hopes for ideal friendship, physical love, and a teacher-student relationship; this remarkable form of union was standard practice at the time, and was subject to its own rules of courtship. It is these rules (and tricks) that Socrates agrees to demonstrate for Hippothales in a conversation with Lysis, the boy Hippothales is trying miserably to court. Thus, the dialogue as a whole rests within this framework of the rational demonstration of passionate love.
Second, this quote is remarkable for its portrayal of Socrates as something quite other than a searching philosopher who sometimes cracks a joke. Specifically, Socrates's powers of emotional deduction are brought center-stage, and are implied to stem both from "the gods" and from a lifetime of experience. In this dialogue, Socrates is both a passionate thinker and a cool, calculating lover. It should also be pointed out that Benjamin Jowett's translation of the above quote is a bit too frugal: Eugene O'Conner amends "these sorts of affections" to "the (male) lover and the (male) beloved."