King Peleus doesn't quite understand why his son Achilles, who's turned away so many young companions, should've fixed upon Patroclus. It's not just Peleus who'd like an answer to this question; Patroclus would too. As he candidly confesses, he has nothing to offer Achilles and doesn't understand why he should've made him such a charity case.
Achilles's answer is that Patroclus is “surprising.” Neither Peleus nor Patroclus understands Achilles's somewhat cryptic response. And as Achilles doesn't care to elaborate, they're left to guess precisely what it is that he means.
On the face of it, it would appear that what Achilles is getting at is that Patroclus has nothing in common with the normal run of foster boys who pass through the palace. Patroclus describes himself as “small, ungrateful, and probably cursed.” Peleus says that “The boy is an exile with a stain upon him.” And yet this is what makes him so appealing to Achilles. He's different, strange, unusual, and not the kind of boy Achilles would normally expect to encounter.
Boys are forever swarming around Achilles and showing off, desperately hoping to be chosen as foster children. But Patroclus isn't like that, and this makes him stand out from the crowd. Like Patroclus, Achilles is somewhat at odds with his surroundings, and in this unimpressive looking boy, Achilles perhaps sees a kindred spirit.