This could be analyzed by looking at 3 of the main characters: Romeo, Juliet and Mercutio.
Romeo is portrayed from the beginning of the play as a fickle young man. Even the Friar suggests this after Romeo meets Juliet, and the Friar cannot believe Romeo has dropped his previous obsession, Rosaline, so quickly. One of Romeo's greatest faults is his thoughtlessness. Though one could argue that his actions were motivated by something deeper than thoughts - they were motivated by love, if he was really thinking of Juliet and not just acting upon his feelings and hormones, he would realize that pursuing her so rapidly and so seriously was putting her in jeopardy.
Juliet, too, makes her own share of mistakes, though. By sneaking around behind her parents' back, she is asking for trouble. Capulet's reaction, as well as Lady Capulet's reaction, to Romeo's presence at their party the night that Romeo and Juliet met, suggests that perhaps they are not so firm in their hatred of Romeo, as they are in hatred of Romeo's family. They knowingly let Romeo stay. Perhaps if Juliet would have slowly tried to win her parents over to the idea of being with Romeo, they wouldn't have been as against it as she suspected.
Finally, we have Mercutio, the most impulsive of them all. He says what he wants, when he wants to say it and to whomever he chooses. He is hot tempered and reacts quickly and without much thought to any and all attacks on him or his friends. When Tybalt shows up, it only takes a few words to get the men to draw their swords. This decision ultimately ended in his own death, followed closely by Tybalt's. Though Mercutio sort of places the blame on Romeo when he asks "why cam you between us?", if his temper had not been so hot, he may not have died.