In Romeo and Juliet, what is the meaning of ". . . young men's love then lies / Not truly in their hearts, but in their eyes"?

Friar Lawrence makes this statement in Act II, Scene III after Romeo tells him:

" . . . my heart's dear love is set / On the fair daughter of rich Capulet. / As mine on hers, so hers is set on mine, / And all combin'd, save what thou must combine / By holy marriage."

Friar Lawrence expresses surprise because just the day before, Romeo had professed his love for Rosaline. Today, he wants the friar to marry him to Juliet. The friar calls Romeo's judgment into question and suggests that Romeo is being fickle and making a decision based on appearance and attraction rather than true love.