Why does Jessica ask Tom to touch her face?

At that point in the story, Jessica has come to develop a deep bond of trust with Tom. They are both outsiders, both at school and in society as a whole. Somehow they just don't seem to belong anywhere. Tom is quiet, a little overweight, and painfully shy; Jessica has been badly disfigured by fire and is shunned by just about everyone but Tom. So it seems perfectly natural that they should eventually become close.
Even so, it still comes as something of a surprise when Jessica asks Tom if he'd like to touch her damaged face. She really must trust him a lot to do so. In a way, it's almost like she wants to reveal her soul to Tom. For good or bad, Jessica's facial disfigurement is now a part of who she is. And although Tom doesn't take Jessica up on her offer, there is no doubt that their previously awkward relationship has blossomed into friendship.