love with each other even though their families are bitter enemies.

Romeo and Juliet lose all common sense because of the love they have for each other. The fact that their families are brutal rivals may even attract Romeo and Juliet to one another because it adds excitement and adventure. The forbidden fruit if you will.

And to make things even worse, they both die because of stupidity.

In Romeo and Juliet, Friar Laurence is probably Shakespeare's voice for his negative view of the impetuousness of young love:

These violent delights have violent ends,/And in their triumph die, like fire and powder/Which as they kiss consume, The sweetest honey/Is loathsome in his own deliciousness,/And in the taste confounds the appetitie.  Therefore, love moderately, long love doth so,/Too swift arrives, as tardy as too slow. (II,vi,9-20)

This motif of the haste and poor judgment of the young lovers as responsible for their fates points to the detrimental effects of "violent delights."  In only three days, the lovers, meet, fall in love, get married, are separated, and then die.  Neither of the lovers stops to reason anything out.  When Romeo, for instance, hears that Juliet is dead, he rushes to buy poison and hurries to the family crypt without trying to find Friar Laurence and ascertain what has really happened.  There is little doubt that the haste of Romeo and his love, Juliet, is tragic, indeed.

If you take a look at Act II, scene iii, the Friar criticizes Romeo for turning on a dime from being infatuated with Rosaline to now loving Juliet. This demonstrates a negative view of love because it proves that for Romeo,

Young men's love then lies not truly in their heart's but in their eyes.

This is a significant point because it demonstrates a problem of all attempts at love, getting stuck on looks. We all do this sometimes, and it's dangerous because there is so much more to a relationship.

Romeo's comments about Juilet in Act I, scene v demonstrate this negative aspect as well.

I guess I would say that you can argue that Shakespeare is saying in this play that love makes people stupid -- it makes them do things that are really completely destructive.

If Romeo and Juliet had been thinking, they would have gone their separate ways.  With their families hating each other, they should have known that their love could never work out.  But instead, they went ahead and got married secretly.  Instead, Juliet tried to fake her death.

Because they were in love, they took silly chances that ended up with both of them dying.

In his verbal exchange with Romeo in Act II, Friar Lawrence cautions Romeo who wants the priest to perform the marriage ceremony immediately for him and Juliet, arguing that he does not care what may happen as a result:

ROMEO. Then love-devouring death do what he dare,
It is enough I may but call her name. (2.6.7-8)

FRIAR LAWRENCE. These violent delights have violent ends.
And in their triumph die, like fire and powder
Which as they kiss consume.               (2.6.9-11)

Certainly, violent love, as Friar Lawrence terms it, is a motif that weaves together much of the action of Romeo and Juliet; moreover, it is this unbridled love that creates a turbulence of passion which, in turn, engenders further violence and death. These two elements of violence and death act together as a negative force upon all the characters.

Here are the different ways that the motif of violent love as a negative force affecting all the characters is exemplified in the play:

• The play opens with violence as even the servants of the houses of Montague and Capulet wish to duel in the first scene
• Lord Montague and Lord Capulet rush out into the fray of the first scene, wishing to do violence to each other and their houses [houses is an implied metaphor for the family and servants of both Montagues and Capulets]
• When Tybalt sees Romeo at the ball held for Juliet, he wants to kill Romeo out of his love for Juliet and the family.
• In Act II, Scene 2, Romeo risks his life and violent action against him to stand beneath the balcony of the daughter of his enemy.
• Romeo and Juliet risk death by marrying each other. (Act II)
• Mercutio and Tybalt accost each other with hatred born of their love for Romeo and Juliet respectively. (Act III)
• Romeo attempts to demonstrate his new love for the Capulets after marrying Juliet, but it acts instead as a negative force, fomenting Tybalt into a more violent action than originally intended against Mercutio. Angered further by Romeo, Tybalt takes advantage of Romeo's standing between them and strikes a fatal blow to Mercutio. (Act III)
• In retaliation against Tybalt and because of his love for Mercutio, Romeo then slays Tybalt. (Act III)
• Romeo's murderous reaction against Tybalt has the negative effect of condemning him to banishment to Mantua. (Act III)
• This banishment of Romeo because of his violent love causes Juliet untold grief and she, then, resorts to excessive and desperate plans when her parents want her to marry Paris in order to change her thinking and console her. (Act III)
• Juliet's love for Romeo drives her to commit the desperate act of drinking from the vial given her by Friar Lawrence which will make her body simulate death so the priest has time to work out a plan for her and Romeo. This is an act that later leads to negative results. (Act IV)
• Because Juliet is presumed dead and fate keeps Romeo from learning the truth, in his irrational love and violent emotions, Romeo purchases poison from a poor apothecary and goes to the tomb of Juliet. (Act V)
• Into this tomb he enters and encounters Paris. Out of his violently possessive love for Juliet, Romeo then kills Paris. (Act V)
• When he finds Juliet, whom he presumes dead, Romeo in an act of desperate and violent love wishes to join her in death and drinks the poison. (Act V)
• After Juliet awakens, she, too, reacts with violent emotion and snatching Romeo's dagger says, "O happy dagger!/This is thy sheath. There rust, and let me die" (5.3.174-175).
• After the death of her son, Lady Montague dies, and, of course, the other parents grieve and are never the same. (Act V)

At the end of the play. Prince Escalus remarks upon the negative force of love:

For never was a story of more woe
Than this of Juliet and her Romeo. (5.3.325-326)