When you can't stop thinking about the other person, and when you think about them you're filled up with happiness and excitement, and you miss them constantly (even if you just saw them an hour ago), and when you're around them (as other people said) the world seems different, and everything's centered on them - and sort of intense blissful feelings and joy fills you, just by being in their proximity, and you're ultra sensitive to even the tiniest touch from them (just like brushing hands or something) - I think the last one is a good indication, because your senses actually change.
Hard to describe, really.
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Into the dark...