I don't know if being "in love" is a thing that actually exists
What do you mean? There's a word called love and there's a reality that people refer to with the word. Words are replaceable.
I'd like to know if it corresponds to an internal state that I have experienced.
The problem is the same hormonal process doesn't necessarily feel exactly the same in everyone's body. I think it would be more reliable to inspect your thoughts and your behavior towards the person and compare them to other people in love.
In particular, how does romantic love differ from friendship?
That's like asking how hot differs from cold, perhaps equally inexplicable, if you're asking about being in love. If you would've experienced it, you'd know. More reasonable people look mainly for friendship in their life partners and therefore might use the words love and friendship interchangeably. They probably wouldn't talk about being in love however.
As a data point I've been in love several times, also reciprocically. It hasn't made me happier on the whole, since the state makes me short sighted and neglect more lasting sources of happiness, and therefore I'm not seeking it anymore.
I think it would be more reliable to inspect your thoughts and your behavior towards the person and compare them to other people in love.
Exactly. But I don't know what the thoughts of other people are and I have reason to think that my external behaviors will differ from others, even if we are both motivated by the same feelings.
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