whose dialogue could be understood to communicate many different meanings, dependent upon the viewpoint of the listener! All of the above posts are demonstrations of this...

All and yet none of these answers could be correct. By virtue of your question, we're considering that writers "seek to reveal some aspect of human nature to us." Certainly, literature reveals traits about humanity and holds them up to us like a mirror. And, just as one's reflection in a mirror includes each detail of our appearance down to our very pores if we get close enough, the portraits that literary works portray are extremely complex. (They, too, also get more complicated the closer we look!) To say "the main theme of Romeo and Juliet is ____" or "Shakespeare is trying to teach us this: ____" is to oversimplify the intensity and nuance of the work. The best we can do is identify a particular theme, like the Free Will vs. Fate idea that e-martin mentioned, and track its development through the plot via specific illustrative examples. But a true scholar would be aware that there are multiple themes, and it is the interplay/tension of those ideas that, along with the reader's personal experience and interpretation, result in the message, effect, and therefore "teaching" conveyed by the text.

Shakespeare is highlighting the explosive energies that intense passion can bring. We see this in the love between Romeo and Juliet, the feud between the Montagues and the Capulets, the anger of Tybalt and the excesses of Mercutio. Intensity of passion is what makes life exciting, but can have tragic consequences.

This play, like Macbeth, seems to explore the notion of "free will versus fate". Though Romeo and Juliet choose to act as they do there is also a strong implication that the two are fated to their particular doom. 

Here the doom is not supernatural but completely natural. It is a doom born out of human nature. In Macbeth the idea of fate is represented by supernatural forces (the witches).

In both plays an outcome is defined before the characters have made their decisions, raising a question as to how avoidable the outcome truly is, how much an individual can change his/her fate, and how set an individual's path may be by forces outside of his/her control. 

Shakespeare is saying many things about human nature throught Romeo and Juliuet. One of his main themes is young love and how moving too quickly can lead to a tragic end. Shakespeare makes it clear that Romeo is too immature to truly love Juliet the way he says he does, and his haste in his actions lead to the death of both of them. This theme is, of course, entertwined with other themes, such as adult ignorance and irresponsibility. Yes, Romeo and Juilet were too young and didn't know each other long enough to really be in love, but the adults should have been more responsible and paid more attention to what their children were doing.

One good way to determine a writer's message, or theme, is to look at how characters change over the course of a story. Romeo and Juliet themselves don't change much, they are impetuous youngsters all the way through. But at the end, as a result of the tragic events, their families, the Capulets and the Montague's, vow to end their feud.

So the theme, which could be stated in many possible ways, should have something to do with the fact that sometimes it takes a significant tragedy to overcome a problem (the feud). You could also focus on the idea that parents and families can be blind to their children's needs when the are distracted by worldly concerns.

Another theme is the over-whelming nature of new love. Romeo and Juliet are swept off their respective feet by love at first sight and this emotional impact affects all of their choices and actions from there on out. They believe in their love and their ability to have their love be able to overcome all of the hurdles their relationship faces.

I think that what he is saying is that human beings are too impulsive and ruled by their emotions.  We see this in practically every character in the play.  So many characters are themselves harmed (or harm others) because they cannot prevent themselves from acting based solely on emotional impulse (as opposed to calm reasoning).

I think Shakespeare could be trying to say several things. One might be a commentary on the foolishness of squabbles like that between the Montagues  and the Capulets, which have the effect of consuming the two lovers, as well as Mercutio, Paris, and Tybalt. Shakespeare may also be trying to suggest something about love, though I'm not clear as to what that is, because I've never been sure about how he views their deaths at the end. In a sense, they die rather than yielding to the world, but in another sense, their forbidden love can only end one way. Fate is also a strong theme in this play, as the actions of the characters seem to lead step by step to the final tragedy.