Symbolism, Imagery, Allegory
It's like this: Deanna's Jason's best friend, but Lee's his girlfriend. So when Jason gives Lee his favorite Metallica hoodie, it's like a knife to Deanna's heart every time she sees Lee wear it:
The hoodie meant there had been a moment, Lee and Jason alone, the sweatshirt changing hands. Did he give it to her? Did she ask him for it? Did she pretend to be cold, wrapping her arms around herself, so that he'd offer it? (2.43)
The Metallica hoodie reminds Deanna of the kind of love and intimacy from which she's excluded—partly because of her reputation, and partly because she was dumb enough to introduce her two best friends.
Later, after the encounter with Bruce at the mall, Deanna goes to Jason's house and sees the hoodie in his room. She wants it, wants to smell Jason's scent in it (even if it also means smelling Lee's "bargain shampoo"), so she pretends to be cold. But, as she tells us, "He dug around in his closet, barely taking his eyes off the TV, and handed me a clean flannel shirt that only smelled like laundry soap" (9.142).
If handing out shirts is Jason's way of showing love, he's just dropped the ball. Even though Deanna continues to wear the shirt he gave her, it's not the same as the hoodie, just like Jason's love for her isn't the same as his love for Lee.