For me it was a feeling I experienced that was unique to that person. We held a connection that we could tell how we feel without words through text message as well as in person.
You forget about your problems when in the presence of this person, and you smile at the thought of them when you're away. You stop thinking about yourself and your needs, and focus on making them happy, because that's what you truly want.
Someone with whom you can be yourself, where nothing feels forced and progresses naturally on both sides.