Good question. It's often been noted that Romeo sees Juliet as something visually pleasing first: it is, of course, love at first sight at the Capulet party:
O, she doth teach the torches to burn bright!
It seems she hangs upon the cheek of night
Like a rich jewel in an Ethiop's ear—
Beauty too rich for use, for earth too dear!...
Did my heart love till now? Forswear it, sight!
For I ne'er saw true beauty till this night.
So far, Romeo hasn't actually spoken to Juliet. Yet Shakespeare goes to some measure to suggest that, when the two actually speak, there is a profound, romantic connection that goes way, way beyond simply a physical attraction. When the two first meet, their lines together form a perfect sonnet: a clear signal of a fusion which transcends simple physical attraction.
Moreover, when the two talk in the balcony scene, Romeo is desperate to interpret her looks, but also her words (or lack of them!) as meaning something. He is desperate to ascertain whether Juliet loves him:
She speaks, yet she says nothing. What of that?
Her eye discourses; I will answer it.
Romeo doesn't see Juliet just for her looks. Yet it's worth remembering that their love goes way beyond words, and words, indeed, are irrelevant to it:
O Romeo, Romeo! wherefore art thou Romeo?
Deny thy father and refuse thy name!
Can you express true love in words at all? Can you describe why you're in love with someone?
Many argue that Romeo is immature, fickle, and overly emotional and that his love for Juliet is merely physical. In fact, Romeo falls for her when he first sees her and vows his love without even speaking with her. Ironically, minutes earlier, Romeo was completely in love with Rosaline for whom he vowed his eternal love.
His love for Juliet’s beauty is apparent from the beginning. As Romeo observes Juliet from afar, he says to himself, “Did my heart love till now? Forswear it, sight!/ For I ne'er saw true beauty till this night." Here, he bases his love for Juliet on her beauty. Later, Romeo furthers his obvious admiration for her looks by comparing her to several beautiful things: "It seems she hangs upon the cheek of night like a rich jewel in an Ethiope's ear."
When faced with banishment, Romeo’s only thoughts are of not being with Juliet, and he cannot bear the fact that he will not be able to look at her beauty again: "Tis torture, and not mercy. Heaven is here Where Juliet lives, and every cat and dog And little mouse, every unworthy thing, Live here in heaven and may look on her, But Romeo may not." Again, here looks are foremost in his mind.
Even when he sees Juliet for the last time in her tomb, Romeo cannot help mention her beauty: “"O my love, my wife! Death, that hath suck'd the honey of thy breath Hath had no power yet upon thy beauty." His last thoughts are of her looks.
For an assignment, one of our editors uses a quote from Carl Jung which seems germane to Romeo's attraction to Juliet, as well:
The meeting of two personalities is like the contact of two chemical substances: If there is any reaction, both are transformed.