One idea about love that Shakespeare expresses in Romeo and Juliet is the fickleness of love. Shakespeare especially expresses this idea through the language Friar Laurence uses and Romeo's characterization in Act 2, Scene 3.
Friar Laurence uses many metaphors to express his shock to learn that Romeo has so suddenly switched from loving Rosaline to loving Juliet, but more importantly, to express his view that Romeo is far too young to understand what love really is. One example of a metaphor Friar Laurence uses is that he compares the tears Romeo shed for Rosaline to salt water and says that he has used the salt to season love, as we see in his lines:
What a deal of brine
Hath wash'd thy sallow cheeks for Rosaline!
How much salt water thrown away in waste,
To season love, that of it doth not taste! (II.iii.70-73)
However, the most important line in the extended metaphor is, "To season love, that of it doth not taste." What Friar Laurence is saying here is that Romeo is seasoning love with the salt from his tears, but he actually has not really tasted love. In saying that Romeo has not tasted love, he is saying that Romeo is far too young to truly know what love is.
Further proof that Romeo is far too young to know what love is can be seen in his characterization. Especially in this scene, Romeo is characterized as a giddy young boy. In this scene, as he greets Friar Laurence, we can hear his boyish energy and excitement over what has just transpired between he and Juliet in the famous balcony scene. We can especially hear it when Romeo speaks in riddles to answer the friar's question about where he has been, such as:
I have been feasting with mine enemy,
Where on a sudden one hath wounded me
That's by me wounded. (50-52)
We can also sense Romeo's characterization as a giddy young boy when the friar addresses him as things like, "my good son" (48). Finally, we can most certainly hear Romeo's youthful, eager giddiness when he states that he is in such a hurry to marry Juliet, such as in the line, "O, let us hence! I stand on sudden haste" (96). Romeo being characterized as an eager, giddy youth helps prove Friar Laurence's point that he is far too young to really know what love is, and Friar Laurence rightly sees Romeo's fickleness as a result of his youth and his inexperience with real love, which is one of the ideas concerning love that Shakespeare explores.
Different sides of love are explored through imagery in the first three acts.
Alternately at different times in the first three acts, love is described as terrible and wonderful.
In the first scene, Romeo has been dumped. Therefore he describes love as difficult to grasp.
Love is a smoke rais'd with the fume of sighs;(190)
Being purg'd, a fire sparkling in lovers’ eyes;
Being vex'd, a sea nourish'd with lovers’ tears. (Act 1, Scene 1)
The metaphors describe love as Romeo sees it. Love is great while you have it, but terrible when you don’t. He describes himself as sinking under “love's heavy burden” (1:4), which is another metaphor. Love also “pricks like thorn” when a simile is used.
Oh, but love can be a wonderful thing too! Romeo and Juliet use beautiful images to tell each other how much they love one another.
With love's light wings did I o'erperch these walls;(70)
For stony limits cannot hold love out,
And what love can do, that dares love attempt.
Therefore thy kinsmen are no stop to me. (Act 2, Scene 2)
Romeo compares himself to a bird, using the metaphor of wings. He is light from love, and can get to Juliet or fly away.
The use of figurative language to describe both sides of love makes Romeo and Juliet's relationship more nuanced. They can express their love for each other beautifully, and through metaphor are able to tell each other what they feel.