"Romeo and Juliet" is a tragedy written by William Shakespeare first published in 1597. It tells the romantic story between young Romeo and Juliet, two lovers whose families are mortal enemies.
Romeo's attitude toward love changes during the story-line. At first his attitude toward love is based on chasing a physical experience rather than a more deep one. Once he meets Juliet, this attitude changes drastically, as he starts to experience a deeper and more meaningful love feeling towards her, and becomes seriously about their life together as lovers. His attitude towards love is an intense and positive one.
On the other hand Mercutio's attitude toward love is a much more pessimistic and detached one than Romeo's. He is a practical character, and he views love as something annoying and meaningless.
Mercutio acts as a foil for Romeo because their views are completely the opposite. While Romeo is a romantic whose conception of love takes over his whole personality, Mercutio does not take love as something seriously. The foil works as a reinforcement of Romeo's intense feelings for Juliet, Mercutio's negativity when it comes to love just highlights how Romeo's viewpoint is the complete opposite, shows how much he is in love with Juliet.