It's when you wanted that someone, but you also want them to be happy. It's when you start to see beyond how you feel about that person and how that person make you feel. It's like when you start seeing that person as a different person, amazing in and out. You love that person for who he/she is.
If you just want that person, you will only love the good side of that person. While if you truly love that person, you will embrace the flaws in him/her too.
You will always want that person to grow, thrive and be happy. It's very less about you and always more about that person. You will start to put that person before you.