The most dominant theme in Shakespeare's Romeo and Juliet is violent passion overcoming reason, or being used instead of reason. Violent passion takes the form of both passionate violence, such as hatred, but it also takes the form of violent, passionate love.
One example of passionate, violent hatred overcoming reason is, of course, seen in the long-standing family feud between the Capulets and Montagues. In the beginning of the play, a fight between the two families' servants turns into a whole-city riot. Not only that, Prince Escalus points out that this is the third time their feud has caused a whole-city riot, as we see in his lines,
Three civil brawls, bred of an airy word
By thee, old Capulet, and Montague,
Have thrice disturb'd the quiet of our streets. (I.i.85-87)
Prince Escalus's lines also point out the very important fact that these whole-city wars are started by "an airy word," or trivial comment, exchanged by Lords Capulet and Montague. The fact that Lords Capulet and Montague permit all-out wars to be started from trivial comments shows us that they are allowing their passionate, unjustified hatred for each other overrule their common sense and reason.
A second example of passion overriding reason can, of course, be seen in the actions of the two lovers, Romeo and Juliet. In the famous balcony scene, Juliet makes it known that she is hesitant to exchange vows with Romeo so suddenly. We see her declare,
Although I joy in thee,
I have no joy in this contract to-night.
It is too rash, too unadvis'd, too sudden. (II.ii.122-124)
Even though Juliet thinks it is unwise to marry so hastily, she allows herself to be persuaded by Romeo. Thus, she allows both her own and his feelings of passion override her rational thoughts. Sadly, the couples' rash, passionate decision to marry so suddenly and secretly helps lead to the couples' death.