She doesn't want to get serious because she doesn't know how to handle serious. She's anxious over it and wants to resolve it as easily as possible. Immature.
Assuming vampire biology works similarly to humans, judging by her appearance her brain has the maturity of a teenager. No matter how old the consciousness, it's still limited by the hardware(brain).
I'm a the only one whose last page is half black ?
Anyways I read it somewhere else and I could be completely wrong but I don't feel like they will end together. More like what is said on that last page they were more like friends since Seri was bored of people loving her and thus she enjoyed speaking with someone who wasn't really interested in romance at the time because he has just gotten dumped by his longtime girlfriend. But then the guy ended in love with her anyway and so they lost that relationship which may be why she is angry toward him and why she want to kill him.
@Mivas Unfortunately, she spilled the beans about her being a vampire. Even if he realizes that what they had was not romantic, and she never drank his blood, his head is on the chopping block. Good work, Seri!