In Romeo and Juliet, is there a simile showing how they long for each other but they can't be together due to their families' feud?

It seems that the best place to look for such an example would be the first balcony scene, in Act 2, Scene 2, before Romeo and Juliet have decided to pursue their love for one another. A close example would be the famous "rose by any other name" line that Juliet speaks out to what she thinks is the empty night:

"That which we call a rose
By any other word would smell as sweet.

So Romeo would, were he not Romeo called,
Retain that dear perfection" (2.2.43-46)

This example is really more of a metaphor than a simile, since it doesn't use the words "like" or "as" (though the word "so" in this case is basically the same thing). Still, it compares how a rose would still smell the same if we called it something else, just as Romeo would be the same person even if he had a different name. She is saying this, of course, because with a different name, he would belong to a different family and they could be together.

Another example comes in the next scene, when Romeo comes to Friar Lawrence to ask for his help. He explains the situation to him (confusingly), saying:

"I have been feasting with mine enemy

Where on a sudden one hath wounded me,

That’s by me wounded. Both our remedies

Within thy help and holy physic lies." (2.3.49-52)

Here he is comparing the love he and Juliet feel for one another to being wounded by each other. This is a common metaphor (sorry, this one isn't a simile either) for love, but it is all the more poignant because he prefaces it with the statement that he was dining with his enemy (the Capulets). This metaphor hints at the problems the family feud will have for Romeo and Juliet without stating them outright. 

Sorry I couldn't find any similes, but hopefully these metaphors work well enough!