I can't pretend to know what it's like to be in love, but I imagine you just want to be with that person, stick by their side. It ends up going deeper than just wanting someone you find sexually attractive and have chemistry with, it becomes unique to that person. That person is all you want and you just need that close, special connection with them; physically and emotionally.
But hey, what do I know. I'm just a single guy who's never been in love.