Romeo oh Romeo, why so whiney Romeo? When the audience first meets Romeo, he's in the most depressed funk ever. The love of his life, Rosaline, has just told Romeo that she would rather live a celibate life than be with Romeo.
Then she hath sworn that she will still live chaste?
She hath, and in that sparing makes huge waste,
Those lines crack me up every time. I would have loved to have seen Romeo's face when she told him that.
Benvolio tries very hard to console Romeo, but eventually Benvolio gives Romeo the "there's plenty of fish in the sea" talk. He tells Romeo that there are plenty of other beautiful women for Romeo to woo and fall in love with.
O, teach me how I should forget to think.
By giving liberty unto thine eyes;
Examine other beauties.
During the next scene, Romeo and Benvolio encounter an illiterate servant that is supposed to deliver Capulet party invitations. He asks Romeo to read the list to him. Romeo quickly discovers that fair Rosaline will be there. Romeo decides to go to the party in order to lay his eyes upon her some more. Benvolio, on the other hand, tells Romeo to go to the party so that Romeo can see way more beautiful women than Rosaline.
Go thither; and, with unattainted eye,
Compare her face with some that I shall show,
And I will make thee think thy swan a crow.
So what happens? Well, Benvolio winds up being correct, because Romeo spies fair Juliet and is married to her two days later. Never a thought of chaste Rosaline again.