Sometimes we do not know we are in love, until what we love the most is gone.
Sometimes we think we are in love, only to find out it was not what it seemed, we just loved (or lusted) after it.
I think the best way to describe being in love is when you look at that one certain person and you realize how perfect their imperfections are, how you love every little detail about them, how you imagine yourself growing old with them, and the reverberating effect of that lasting first kiss, all at one time.