I think Romeo is a firm believer in what he would call love. The difference between his experience with Rosaline and Juliet has to do with rejection or acceptance.
It takes two to experience a loving relationship. If there are not two involved, it is just then lust, infatuation, or longing. The concept of love insinuates two collaborative parties... at least in a male-female consentual relationship.
You may be referring to Romeo's comments that love is "rough, rude, and it pricks like thorn." Even though Romeo refers to love I don't think this is what he means, but it is his perception of love at that time because that's how he labels what he feels for Rosaline. His comments are a result of his feeling rejected. He was hurt.
Upon meeting Juliet for the time, he thinks he has never seen anything so beautiful. Since when is love based entirely on looks? Let me tell you, since teenagers have inhabited the earth. Romeo's perception changes, but it goes from one teen issue to another.
For most people, falling in love and being in love are two different things. I think that this is true for Romeo as well. Since he meets Juliet in Act One, this should still be considered "the start of the play," and it will take the culmination of all the other events of the play:
- Juliet putting everything on the table in the balcony scene and asking him to marry her
- the fateful meeting on the street with Tybalt
- the death of his friend Mercutio
- his banishment
- Juliet's "death"
for Romeo's love to mature from infatuation to something deeper.
All these events are what test and develop Romeo's love, just as love in real life is tested and deepened. I think that Romeo does fall in love with Juliet at the Capulet home in the same manner that he was "in love" that very same afternoon in the street with Benvolio. How could he not? Nothing has happened yet to change and test him. Those events are to come in Acts Two, Three, Four and Five.
Romeo, like another character in Shakespeare, Orsino (Twelfth Night), is in love with the idea of being in love but he really doesn't understand exactly what love is.
Young love is strong and passionate, whereas mature love is based on more than a physical attraction. If couples are lucky, they discover each other in the passion of youth and this matures to real love.
At th beginning of the play, Romeo is moaning and groaning because his love for Rosalind isn't reciprocated . Benvolio and Mercutio both give him a hard time about his being in love. The whole idea of crashing the Capulet masque ball is so Romeo can compare Rosalind to other, perhaps more eligible, young Verona ladies. Actually, Benvolio's plan works but Romeo picks Juliet.
Although he doesn't know it, he has fallen love with someone more unattainable than Rosalind...or has he? Juliet responses to him.
The big question here is this, does Romeo fall in love with Juliet because she is the most beautiful girl in the room, outshining Rosalind, or because he has learned what is really love.
Since once he finds out who she is, he puts it all on the line for love when he trespasses onto the Capulet property, I would have to say that he has grown. Once he and Juliet meet, they connect. During their first lines to each other, these two characters connect. Their shared sonnet shows how they are on the same page, so to speak.
Romeo learned that love is a two way street, not a one way street and with Juliet, he changed directions, so I would have to say that he grew from Act I, scene 1 until Act I, scene 5. He grew; he gets it. He understands what love really is. He was totally committed to her. By the end of the play, he chooses to die rather than live in the world without Juliet.
Unfortunately, their love never had time to grow and fully blossom.