One Love is capitalized while the other love isn't,
A person can say I love you, which is a common every day thing...I love the way your new hairstyle looks, I love your new car, etc etc...but when you look at that special person n say ' I am so much in Love with you '...your Love means more to me then stars in the sky...
Saying I love you is easy, but saying I am in Love with you is a lol bit different...