AgentZigzag It was quite complicated reasons when he actually got into it, and I asked him a lot of questions (like to understand the roots of these things).
He equated fire with some sort of Divine being, or entity with massive power over him, "like the closest you can get to God" - in his words. He also had massive guilt over things he'd perceived himself as having done wrong (stemming from severe criticisms/chastisement in childhood).
Without an adult or authority figure to 'punish' him anymore, he sought a kind of ultimate relief by offering himself up to "Fire" (personified) as a replacement. Being burned and suffering pain was his punishment, and being allowed to live and not entirely consumed by the fire was his salvation - his kind of "OK" sign to keep living from God.
He was entirely aware that his practice of burning himself (which he did when not helped by others) could end up killing him, and in fact that was a necessary part of the experience for him, that death could be possible, and pain inevitable.
He was a very, very tortured individual, and I do still think of him actually because I wish I could have reached out to him in some way that would have made a difference. But it was like he was too far gone, or too far away, and so desperately clinging that I had to be another person to cut him off completely.