An example of a romantic quote for a girl you like is from the book "The Crown of Embers" by Rae Carson: "I love you the way a drowning man loves air. And it would destroy me to have you just a little."
Another quote you can use to express how loving someone changes you is, "I love you, not only for what you are, but for what I am when I am with you," by Roy Croft. A romantic quote about love at first sight is from Shakespeare's "The Tempest": “Hear my soul speak. Of the very instant that I saw you, Did my heart fly at your service.” Parts or whole sonnets written by Shakespeare can be used as romantic declarations, such as "Sonnet 161" or "Sonnet 45."