In addition to the excellent answers above, there is one other element that begs us to question Romeo's feelings. At the beginning of the play, Romeo is depressed because the love he feels for Rosaline is not being reciprocated. In fact, it is Benvolio who urges Romeo to attend the Capulet's feast as a way of "getting over" Rosaline:
At this same ancient feast of Capulet's
Sups the fair Rosaline whom thou so lovest,
With all the admired beauties of Verona:
Go thither; and, with unattainted eye,
Compare her face with some that I shall show,
And I will make thee think thy swan a crow.
Romeo, of course, wants nothing to do with anyone other than Rosaline. In fact, he responds to Benvolio with the following:
Just before the feast, Romeo is still convinced that Rosaline is the only woman in the world for him. Of course, one glimpse of Juliet changes all of this instantly and she becomes his new "sun."
By the next morning, when Friar Lawrence questions whether or not Romeo has spent the night with Rosaline, his response is:
We must question how true and pure Romeo's love for Juliet is because of how quickly he dismisses and even forgets his love for Rosaline. If we attend to another line of the bard's great advice--"what's past is prologue"--it could be argued that Romeo will eventually forget Juliet as quickly as he did Rosaline.
I am not entirely certain that I could make a case for the love being impure. I think that this descriptor involves a level of conduct or motivation that I am not sure can be found in the text. I would say that a convincing case can be made that the love experienced might not be "true." It seems like there is a level of superficiality to the affections expressed by both of them. In Romeo's case, his "love" for Juliet seems to be driven through physical sight of Juliet and his own sense of "rebound" from other examples of failed relationships and a general malaise with his social predicament. It seems to me that there is a sense that indicates his emotional state with Juliet might not be "true" love, as much as an infatuation.
Another argument to add to the answer above is that they are so young. Juliet is what, barely 14? Romeo only a few years older than that. Because they've grown up in such wealthy families, neither has had to "survive" or live on their own. They are both spoiled children. Some argue how could they possibly know what true love is? Both are used to getting whatever they want (assumedly because of wealth), neither has a very close relationship with their own parents, and they both lack the life experience our own society would deem necessary to have a successful marriage.
In other words, if Romeo and Juliet did what they did in today's society, it would probably be laughed at, frowned upon, pitied, and expected to end soon.
The fact whether Romeo and Juliet's love was pure or not is often debated. Some people, for example, believe in love-at-first-sight; others do not.
An argument that their love was not pure might be that they didn't know each other long enough to know if it was a long-suffering love, or if it was pure infatuation.
Romeo, in Act I, scene v, certainly demonstrated he was taken with Juliet because of her looks. The friar confirms this possibility in Romeo when in Act II, scene iii he tells Romeo that men care more about women's appearances than their intellects. Juliet even confesses that a commitment between the two of them seems too rash, too sudden, and too ill-advised. Juliet seems to be acting against her better judgment with Romeo because she even worries that he will think she is too easy (Act II, scene ii).
These are several reasons why they may only be acting on infatuation instead of love.