There are a few good quotes from Act II, scene iv.
Here is one Mercutio says to Romeo:
Why, is not this better now than groaning for love?
now art thou sociable, now art thou Romeo; now art
thou what thou art, by art as well as by nature:
This quote shows that Mercutio knows Romeo, he knows what Romeo is all about and the previous depression he watched in Romeo was uncharacteristic. This also shows that Mercutio cares for Romeo's well-being apart from his need for teenage love. He essentially says in the last few lines here that Romeo should be social and fun, it is good for his soul.
Another quote Romeo says of Mercutio is this:
A gentleman, nurse, that loves to hear himself talk,
and will speak more in a minute than he will stand
to in a month.
Romeo says this to the nurse after Mercutio teases her. This quote characterizes Mercutio as a big talker. This is true. He never shuts up. This is interesting in the relationship between Romeo and Mercutio because Romeo obviously has a sounding board and friend who would offer opinion and advice, yet he keeps his friend in the dark about Juliet. This is a problem.