[ what's the best way to describe it ...? giorno tilts his head to think about it for a minute, before replying with, ]

I can create living things from nothing. This includes plants, animals ... body organs.

The reason why it hurts is because it's becoming part of you again. Think of ... a limb, being forced to reattach once more to the point of injury where it was severed from. To have each individual cell and muscle and bone be reattached and grown again like nothing is wrong. It's not technically healing, but more a means of grafting new parts to pre-existing sites.