Act 2, Scene II of Shakespeare's Romeo and Juliet begins with Romeo's famous soliloquy in which he sees Juliet appear at her balcony. Eventually, Romeo's unheard admiration moves to conversation as the two young lovers eventually begin to speak with one another and share their feelings of love.
When Juliet suggests that Romeo should swear that he loves her, the young man is ready to do so. Nevertheless, Juliet comments:
Well, do not swear: although I joy in thee,
I have no joy of this contract to-night:
It is too rash, too unadvised, too sudden;
Too like the lightning, which doth cease to be
Ere one can say 'It lightens.'
Thus, from her comment, it appears that Romeo swearing that he loves Juliet will give her pleasure, but that she does not have the time, at that particular moment, to take any pleasure from his oath, which she describes with the word "contract."
So, I would say that it is not so much their love that is like lightning, but rather Romeo's oath regarding his love that becomes like lightning. Juliet's ability to enjoy hearing Romeo's oath will be very limited because she cannot leave her house to be with him at that particular time. Indeed, in just a few moments, Juliet will hear her nurse calling from within the house and she will have to leave.