You dont just automatically love someone. You slowly learn how to trust them, how to start believing in them.
You want to be with them to the point where you're jealous of anyone else who gets a little of their time. It gets you mad, but you learn to get past it, because you can't be jealous forever. And then you realize, you don't even feel jealousy anymore because you have thin unwavering confidence that this person will never leave you, never betray you and would never pick someone else over you. They make you feel irreplaceable. That's when it hits you.
You really do love each other and it's completely unbreakable.