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This applies equally to getting to know someone in a non-romantic context, and in fact fairly well describes my excitement at meeting someone who I think might be a potential friend. Why is it sometimes feelings of love instead of friendship?
(Note that this is in the context of not really understanding the difference between a friend-relationship and a romantic-relationship.)
I don't think there's a good answer to that apart from some mishmash of happenstance, pheromones, social context, and neurological variation.