Probably through separation of physical and emotional, and with both aspects weakening drastically upon separation. If you want my theory, I’d say that despite them positioning themselves as a superior race and looking down on humans, them all being former humans (along with the vampirism and eternal life being considered a curse even by themselves) ensures that deep inside they view themselves as monsters. The psychological response to it is the longing to be loved and cherished despite being monsters and despite hardly being able to respond in kind with affection of the same magnitude. Ferid, for example, even goes as far as wanting to be hated. It’s a weakness that, at the same time, is their last surviving link to their humanity if twisted. FYI, I’m still debating the hypothetical situation of a vampire (a few centuries old vampire, to be exact) being in some semblance of love and if that would be enough to trigger the desire for something physical, even if it’s just a kiss or an embrace, in them.