So Shakespeare got it wrong?
No, he got it just right. Romeo and Juliet is a parody of Elizabethan love-tragedies; it is meant to be laughed at (and would have been heartily by the audience then). The whole point of the play is that teenagers are incapable of "true" love (with probably some personal baggage from his own troubled marriage to Anne Hathaway in his late teens mixed in, too).
The fact that many school systems (at least where I am) teach this play to teenagers compounds the misinterpretation of the play, as it is very difficult to convince teenagers of anything, especially when it is to tell them that they don't understand something due to their youth. So they carry forward their mistaken view (aided and abetted by incompetent/undertrained teachers in some cases) that they play is somehow about the trials of true love, rather than laughing at how stupid Romeo and Juliet were in the play. Romeo is madly in love with Rosaline at the beginning of the play (swearing his eternal devotion), yet he suddenly is madly in love with Juliet shortly thereafter? The continuous use of haste and imagery associated with it (including allusions to gunpowder and the "explosiveness" of young love)? The stylistic similarity with A Midsummer Night's Dream? All of these point to the theme of R&J: it is a parody meant to illustrate that young love is hasty, impetuous, and dangerously false.
Nope, Shakespeare got it just right...
Its a piece of fiction.