If a guy says these three simple words to a girl and means it, then every second he spends with her, every time he hears her voice, every kiss, every laugh, every joke, every argument- is the best moment of his life. When you really mean it, you'll never get tired of saying or hearing these words. When you're in love, you'd rather hear the other person's voice than sleep. You care more about them than you care about yourself. You'll never let the other say they love you more. You could go on and on trying to define your feelings for that person but you'll never find words to accurately explain.
I can easily define these words because I know from personal experience. I'm in love and I don't care who knows it. I love you Drew.
Taylor: I love you too.
Drew: Well...I love you more!
Taylor: No you don't! Two camels in a tiny car! Ha!
Drew: (grumble grumble...)