What in their environment and in their specific lives push them towards this impetuosity?
The two lovers meet declare their love ,exchange vows of love , and plan to be married all in a matter of hour. They repeatedly demonstrate that they prefer death to separation
Part of what drives each of them into the arms of the other is the element of social dysfunction that envelops them. The reader understands that the Montagues and the Capulets are at war at the outset of the work. Yet, no character really gives a clear understanding as to why each family is inwardly drawn and possess such a great resentment towards the other. It is never fully divulged why each family has this disdain for the other. In some respects, perhaps both adolescents are questioning this, themselves. This would make their plunge into one another highly plausible, for in the other they find one who is questioning why the social order is what it is and in their love, a hopeful transformation of what can be is present.
Concerning the impetuousness of the two lovers in Shakespeare's Romeo and Juliet, there are plenty of details we can infer about their environment that would lead them to behave this way. They are told not to behave this way by their parents and tradition, for example, so, of course, they do. Or, we might say they just plain love each other.
There's another side to the question, though. Remember that Shakespeare is not writing in an age that emphasized realism. People did not speak in iambic pentameter in Elizabethan England any more than they do today. No one is as metaphorically witty as Shakespeare's characters are. Shakespeare was not going for realism. The joys and satisfactions come when you view the play if you suspend your disbelief and accept the fictional world of the drama.
In other words, audiences accept their love and their actions because they want to, not necessarily because everything in the play is plausible and makes rational sense. If one dismisses their love as unrealistic, the play isn't much fun to watch.
In my opinion, what pushes them in this direction is their lack of control over their lives.
In the time when Shakespeare was writing, children had essentially no say over what happened to them. Their parents had the right (as we see happening with Juliet) to tell them who they would be allowed to marry.
In such a situation, you can see why young people would tend to be impetuous. They would have had little chance to practice self control. In addition, they would feel pretty helpless to affect their lives. This would make them feel the need to grab what they could as soon as they could.