There are so many reasons Romeo and Juliet should not get married.
First, there are the obvious problems. Their families hate each other—in fact, they are killing each other when they can—so the twosome has to sneak around and marry secretly. As we find out when the married Juliet is expected to marry Paris, this was not the best of ideas.
Even without that obstacle, there are other serious issues. For example, they have only known each other about a day when they get married. That really isn't enough time to get beneath the surface of physical attraction, let alone get to know the other person.
Further, as Friar Laurence points out to Romeo, he should beware that his passion for Juliet is running so high that it might burn out. Then he would be married—in a society without divorce—to a person he didn't love.
Finally, the two are very young. Romeo's age isn't given (he is assumed to be in this late teens or early twenties), but he acts very much like an adolescent. Juliet is scarcely 14. Early marriages were more common during the time of the play, but both nevertheless seem very inexperienced to be entering into such a serious commitment as marriage.
It would make sense for them to slow down, get to know each other better, and perhaps find a way to approach their parents before rushing into matrimony.