I guess Shakespeare presents a negative view of love by saying that people are essentially blinded by love. When two people are this infatuated with one another it becomes difficult to make wise decisions. All odds are against Romeo and Juliet and yet they still continue to fall madly in 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.