He hadn't been all that worried that Cassidy would be put off too much by a little pain; it's not as if Eccarius is tearing his throat out after all and his fangs are very sharp, as Cass had already observed. So it's a little bit like comparing a sharp knife to a dull one, with the latter hurting quite a bit more than the former.

Eccarius doesn't reply to the quip, his mouth being rather busy as it were, especially now that it's full of that sweet, tasty blood. Vampire blood, the consumption of which is a very heady rush in itself. Sucking Cassidy dry sounds like a plan really, and he finds himself moaning a bit with the pleasure of it.