The vampire didn't love the boy actually. Did he?? I don't think so...

I think he saw him as special but it took them being apart for him to see him that way. Then theres also the whole concept of the heart. My take on it is the seme realised that it meant you could simply choose your heart to be someone special to you rather than having to consider them special right away (like love at first sight)
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I think they're happy after realising their feelings.