I can hardly suppose that you will affirm a man to be a good poet who injures himself by his poetry.
Socrates speaks this line to Hippothales by way of wrapping up his argument that Hippothales is simply flattering Lysis into inaccessibility. Socrates will argue that Hippothales should seek to humble Lysis rather than inflate him. The quote is important because it both recalls the standard Socratic elenchus and shows the unusual form the elenchus tends to take in this dialogue. Socrates's argument is standard in that he has taken Hippothales's high ideals (singing dramatic songs in praise of Lysis's family) and turned them on their head. Further, Socrates does this through a method he often uses in other dialogues: he convinces Hippothales that what he thinks is benefiting him is actually "injuring" him (in this case by making Lysis too proud to accept Hippothales's advances).
All of this is classic elenchus, in a remarkably compact form. Nonetheless, the argument is unusual in its specific elements. Socrates's argument is based almost entirely not on analytic propositions but rather on wisdom about the way people tend to behave in relationships of love. Specifically, he knows that "the more vainglorious [the beloved is], the more difficult is the capture of them." Significantly, Socrates returns to the hunting/capture analogy multiple times in the dialogue, sometimes with regard to the beloved (as here), sometimes with regard to philosophical argument, and sometimes with regard to both (as in the remarkable line that "arguments, like men, are often predators"). Socrates is almost always playful in the dialogues, even, to some degree, on his deathbed in the Crito. But here the very form of the elenchus, in both its context and its content, is fraught with questions of love and desire, and the dialogue is very playful indeed.