Poems are like people (simile)
"I got to thinking that poems are like people."
Ari compares poems to people, because both tend to provoke the same reaction in him. He understands some right away, and others he never understands at all.
Pain like a storm (simile)
"Sometimes pain was like a storm that came out of nowhere"
Ari uses this simile to express the fact that everything can be going very well, and you can be happy, but pain and heartache can sneak up on you. Even on sunny days, a sudden, unexpected rainstorm can come. This is how he feels his life can be.
Love as a heavy object (simile)
"Love was always something heavy for me. Like something I had to carry."
Love for Ari is not something light and pleasurable. It is a burden. Loving someone is a burden. Being in love is a burden. It is as if he has to pick up love and carry it around with him actively, and if he wants to put it down on the ground again and rest, he would lose that love.
Dante's face like a map of the world (simile)
"And it seemed to me that Dante's face was like a map of the world. A world without any darkness."
Dante's face is compared to a map of the world because for Ari it is a face and a world that provides endless opportunities. There are no barriers up and it is totally open. Ari feels as though he has a lot of darkness in his world, and to see only light in Dante's is astounding to him.
High school as a prologue to the novel of life (metaphor)
"Senior year. And then life. Maybe that's the way it worked. High school was just a prologue to the real novel. Everybody got to write you—but then when you graduated, you got to write yourself."
Ari uses the writing of a book as a metaphor for the way in which a person's life develops. Whilst you are a child, your life is written by other people. They form your character, they get to write your storyline, and they are able to write you as the person that they either want you to be or believe that you are. Once you graduate, you are responsible for writing, or re-writing your own narrative; in other words, you are now able to become the person you want to be, not what others expect of you.