Sal has been Miranda's best friend since she was a very tiny girl, and she thinks of him as her other half. One of the phrases the she repeats through the novel is "Sal and Miranda, Miranda and Sal," which reminds us just how closely the two were joined together when they were kids (5.9). Miranda gives us plenty of examples of just how tight the two of were:
One time, when Sal had a fever and Louisa had called in sick to her job and kept him home, the daycare lady handed me my carpet square at nap time, and then, a second later, she gave me Sal's, too.
"I know how it is, baby," she said.
And then I lay on her floor not sleeping because Sal wasn't there to press his foot against mine. (5.11-13)
As small children, Miranda had come to depend on Sal's presence for her very well being. Here she can't even sleep without his foot pressed against hers. While this kind of thing might be precious in a little kid, that level of closeness can be suffocating for a preteen who is trying to grow up into his or her own person – which is why, as the novel opens, Sal is beginning to pull away and no longer wants to be friends with Miranda.
Over the course of the novel, Miranda must try to understand and reconcile herself to the fact that both she and Sal are growing up and can't possibly be as close as they were as children. Especially since Sal would like to hang out with, you know, the guys from time to time. Sal eventually explains to Miranda that the intimacy of the relationship was smothering him – and kept him from making other friends:
"That's the thing, Mira. It wasn't normal. I didn't have any other friends! Not real friends."
Neither did I! I wanted to say. And then I realized – that was his whole point. We'd only had each other. It has been that way forever. (47.27-29)
Though Sal recognized long ago that his relationship with Miranda couldn't last forever, it took Miranda much longer to catch on. By the end of the novel, though, we find that the two have been able to mature their relationship into a less dependent, more balanced friendship:
Sal and I don't wait for each other these days. Not purposely. But if we happen to be leaving school at the same time, if he isn't going to a friend's, or to basketball practice, and I'm not going to Annemarie's or Julia's – or Colin's – then Sal and I walk home together. And we are better this way, together because we want to be. He understood that before I did. (54.1)