Romeo and Juliet's passionate love for each other acts as the fuel which ignites their foolish impetuosity. Thus, the two lovers act against reason, eloping and rashly committing actions that cause domestic strife, death, banishment, deceit, and finally and tragically, death. Amidst the strife and carnage, Romeo, blinded by his pride and love for Juliet, foolishly blames his weakness in causing the death of Mercutio and Tybalt; further he blames fate as his foe when he hears the rumor of Juliet's death from Balthasar. Impulsively, he purchases poison and rushes to the Capulet tomb. There, in his impassioned state, he foolishly kills himself even though he has noticed that the bloom of life is yet in Juliet's cheeks. Then, of course, Juliet imitates Romeo's impulsive act, as well.
I am going to take a different perspective on the play in this response. It is easy for us analysing the play from our contemporary perspective to underestimate the extent of the foolishness of Romeo and Juliet's actions. We live in a culture where it is expected that we will pick our own life partner, but let us remember that at the time of the initial performance of the play, very few people actually had the luxury of selecting their own wife or husband. In particular, for people of Romeo and Juliet's standing in society, marriage was something that your family organised for you as marriage was conducted amongst the upper classes for means of wealth, prestige and alliances. Thus, if we think about it from this perspective, it is clear that both Romeo and Juliet were very foolish to marry in secret and to let their own feelings of love overwhelm other, more important feelings such as their own family responsibility and duty to their houses. Note how Lord Capulet views his own procuring of Paris as Juliet's future husband in Act III scene 5:
How? Will she none? Doth she not give us thanks?
Is she not proud? Doth she not count her blest,
Unworthy as she is, that we have wrought
So worthy a gentleman to be he bride?
From Capulet's perspective, he has been a good father, finding an excellent, wealthy husband who occupies an important position in society. Juliet therefore owes it to herself and to her family to marry Paris. Her love for Romeo has therefore foolishly made her cherish her own wants above those of her family.