If you're inspired by the image then that helps.
Now, my reasons:
1) I imagine that this is like growing a "lava carapace" of sorts - you're using lava to stitch up your wounds and regenerate your body.
2) It's a heart. Hearts stand for love, which can represent multiple things - love, healing, passion, etc... In this case chris went with the healing/passionate aspect of love I'd guess.
It's slightly off theme, but not so terribly off that it would require an element change. Fire is aggressive, but the lava cools every now and then too. I would say it's more of a different interpretation of what fire could do rather than just making something up on the spur of the moment.