I have been asked this question once or twice by women I fell for. I always answered the same: "I don't know", because I have no idea why and, like @diogenes_cask, I don't really get interested in the answer. I also agree that it only made me ascend, sometimes to my greatest despair though, as the Sun burned my wings 🙂
What's sure though is that I always deny it first and have to get pushed to admit both to myself and someone that I developed such feelings. Like @Oxiu said too, it often comes with transformation, the person I see becomes incredibly more beautiful (and more and more with time) than what I would think without being inlove.
Your answers @psysarah15 and @Lianshen are super beautiful and true i think!! But still i think about it too like also what @Simone724 asked because maybe you already have been in love and you thought it was only that you just liked somebody. But maybe it is impossible to know until you really have relationships too.