One of the more hilarious descriptions that Hazel gives us is of the Support Group that she goes to, which is basically just a misguided mess of sentimentality. The Support Group is held in a church basement that is shaped like a cross, and they sit where the two lines of the cross meet. Because of this, their Support Group leader refers to the group as the "literal heart of Jesus."
Hazel, on the other hand, doesn't see the rainbows and butterflies:
So here's how it went in God's heart: The six or seven or ten of us walked/wheeled in, grazed at a decrepit selection of cookies and lemonade, sat down in the Circle of Trust, and listened to Patrick recount for the thousandth time his depressingly miserable life story... (1.6)
The heart of Jesus reminds us that everyone has their own way of finding solace in tough times. Even though Patrick and some of the other Support Group attendees find it uplifting and comforting to be so embedded in the presence of Jesus, Hazel and Augustus just find the whole thing depressing and absurd. For them, the Heart of Jesus is a confining, suffocating place where they are defined by their illness. And that's essentially the last place they'd like to be.