When Romeo and Juliet first meet, Juliet asks Romeo to not swear by "the inconstant moon." Considering that the lovers' encounters are in the night and done in haste, they have, in a sense, involved themselves with this inconstancy.
True love, as St. Paul teaches, is "patient," constant. The love of the loyal family members and friends, as noted in #2 is this constant love. As Friar Laurence cautions, "These violent delights have violent ends." The lesson of true love is that it grows slowly in order to be constant and lasting, not having a violent end.
THink about your question. What DOES Romeo and Juliet teach us? You could take it from the perspective that true love doesn't exist. Also Shakespeare's intense sence of failure and hatred can lead us to understand that love causes the most problems. Fueds and wars start over love, but as in the case with Romeo and Juliet they can also end over love. Hope this helps.
This play teaches us many things about love--for instance, love at first sight is possible (unless your perspective is that these two aren't really in love, but lust, and it ends up killing them). It suggests that true love can heal all wounds--the feud actually ends as a result since the parents of these two attend the joint funeral together peacefully.
It suggests that true love is also loyal. Think of all the friends who died and for whom revenge was sought...Mercutio, Tybalt, Paris...of course, Romeo dies for Juliet, and when she awakes she kills herself rather than live without her Romeo.
Whatever your take on what this play teaches us about love (you can point out both positive and negative lessons), you should be able to give textual evidence to back up and support your stance.
As a supplement to the other editors' answers, one might also conclude that Romeo and Juliet teaches us that true love is not to be tampered with, or else dire consequences will result.
Romeo and Juliet are, obviously, infatuated with one another despite their feuding families. When that love is inhibited by the families, tragedy occurs. The lesson to be learned here is, "Don't mess with true love."