The main difference to me is that Romeo's "love" in Act I, Scene 1 is a sad love because it is not returned by Rosaline. You can argue that it is less mature as well. The event that occured to make Romeo feel unhappy in this act is that Rosaline does not love him as he loves her.
By Act II, Scene 2, Romeo's love is returned, this time by Juliet. He is now happy. This is because he met Juliet at the dance and she clearly loved him as he loved her.
To me, the main linguistic device that Shakespeare uses to contrast the two emotions is seen in what Romeo compares the women to. Rosaline is compared to the moon, which is pale and cold. Juliet is compared to the sun which is warm and bright and which drives the moon away.
The event that caused Romeo to feel this way for Juliet is totally and completely the visual sight of her and the reaction that happens within him upon seeing her beauty. He was instantly mesmorized.
Upon Juliet's returning love, I believe both audiences believe their circumstance to be at the very least infatuation because we see that puppy love in teenagers all the time. That attraction is very physical, emotional and as Juliet comments, "too rash, too sudden, [and] too ill-advised". This is typical behavior for teens no matter the era or culture. At work here is biology and chemisty, neither of which change over time.
Romeo and Juliet is full of little indications about the love and the validity of the love that Romeo and Juliet feels for one another. The main question really is if this situation was to happen today would what is percieved as love then, be love in the eyes of today's society. Here are my thoughts from the question you have posed.
Romeo at the begining of the play is pining for the love of a woman called Rosaline. Or is he? As mentioned above, Shakespeare gives many indications as to what the feelings are between the characters within this play. I believe that Romeo has more of an infatuation with Rosaline and because of his lack of maturity fails to actually love her but instead insults her by offering to pay her money for sex - this is in actual fact prostitution in a modern day society. At this stage in the play he is knew to the idea of love and instead of feeling love for Rosaline he feels lust. Honestly mistaking it for real love- “won’t ope her lap for saint seducing gold”
The contrast between his feelings is vast - it could be seen that Romeo is still too immature to know the difference between love and lust as he falls in and out of love so easily. He falls for Juliet so quickly. In contrast to this he does explain how juliet completes his soul and she is his Sun and “he hasn’t seen true beauty til this night”. This appears to be the real deal for him - he is willing to lose everything for juliet as opposed to some gold for Rosaline.
Elizabethan audiences probably would have believed this love between the two. Today, in my opinion it wouldn't be as believable. I guess you have to look at the social, cultural, historical and political aspects of the time and ask if this is something that was done then? Did people fall for each other like that or was it the romantic ideology that we try to hold now within fictional books such as Twilight with Bella and Edward? Is anything possible and do you believe in Love?
It is all questionable - but Shakespeare does give a lot away as to whether is it supposed to be believed as genuine or not. Maybe have a closer look into the symbolism of his language with Juliet compared to Rosalind. Do you forget someone you love so easily for another?