He wasn't quite sure what he'd expected her to say, but a confession of love was probably pretty low on that list, mostly just because it'd never happened before. He didn't do love. He'd told her that already. He hadn't elaborated on it since they'd decided to try this out. And maybe back then, they hadn't really expected all of these outside influenced either. This was probably moving fast, they were probably finding out a lot about each other in this shit storm a whole lot faster than they would've otherwise.
Generally, when someone told you that they love you, the appropriate, expected response was to say it back. In truth, Alec wasn't entirely sure when the last time he'd ever said those words were. He didn't say them to parents, or family, or friends. He wasn't raised in that kind of environment, where the free declarations of love were common and pretty much a given. Which meant that he didn't actually receive them all that much often either. Or ever.
For his part, he didn't shove her away, or leave her. His expression was probably hard to read, as it tended to get in these kinds of situations. "You're not fucking with me?" He asked. This was probably not a declaration of love for the books. "Because you know you don't owe me this, right?"