What is Romeo's hubris?

In classic Greek tragedy, the hero has a tragic flaw that brings him to his doom. The flaw is generally hubris, or stubborn pride. At first glance, Romeo doesn't seem especially proud. However, he is self-absorbed, which could be considered a type of pride. In the beginning of the play, he is moping around inconsolably because Rosaline doesn't love him. He refuses to consider his friend's analysis that he only loves Rosaline because he hasn't met anyone prettier yet. But, as it turns out, Romeo's friends know him well. As soon as he sets eyes on Juliet, he is consumed with his infatuation for her. She tries to slow him down during their first meeting, but he ignores her mild protests and manages to get two kisses. Later, when they speak on the balcony, Juliet again warns that they are being too "rash," but he starts planning their wedding. Friar Lawrence warns him that he is moving too quickly, but again, he pays no heed. This unwillingness to take time to listen to others is a type of hubris.
The most tragic example of Romeo's hubris, however, occurs during the encounter with Tybalt. Although Romeo tries to break up the fight between Mercutio and Tybalt, after Mercutio is wounded, Romeo forgets his own advice to his friend. He makes this speech:

My very friend, hath got this mortal hurtIn my behalf. My reputation stainedWith Tybalt’s slander—Tybalt, that an hourHath been my cousin! O sweet Juliet,Thy beauty hath made me effeminateAnd in my temper softened valor’s steel.


Rather than thinking rationally, he lets his pride take over. He worries that his "reputation" has been "stained" and that he might appear "effeminate" for not fighting back. He kills Tybalt.

All Romeo's previous foolishness pales in comparison to this. This is the event that sets the tragedy in motion. It is the moment where Romeo displays not just self-absorption but pride, demanding revenge. This is the hubris that leads to the deaths of both Romeo and Juliet.

Hubris is having too much pride or self-confidence, to the point of being blinded to the full picture of reality or to other people's points of view. Romeo's hubris comes from his youth. Like a typical adolescent, whatever is happening to him now is the most important thing in the universe, and whatever he is feeling now is what he believes he will be feeling forever. He acts out of the moment, whether the impulse is destructive or not. He is not one to stand back, consider, and take the longer view.
For example, when he is pining for Rosaline, he believes she is absolutely the only person in the world he could ever, possibly, be in love with. He resists going to Capulet's ball, stating no one could possibly replace Rosaline, until, after a short time there, he falls head over heels in love with Juliet. Then he has to marry Juliet right away, a day after meeting her. There is no waiting, no thinking this through. Likewise, when he returns to Verona to find Juliet dead, he doesn't stop to think. He believes the agony he is suffering at that moment will be his reality forever, and so, with full confidence that he is doing the right thing, he takes his life.