It takes time to gather enough wood, piling it high. The Doctor does it himself, the hard way, his penance for… what exactly? Failing to save his old friend? The Master was the only other Time Lord left; now the Doctor is the last of his kind again. It hurts.
So he builds the biggest bonfire he can, a funeral pyre for his childhood friend, wishing things hadn’t ended this way. The Master could have regenerated. Why didn’t he? That question will never be answered now.
Lighting the fire, consigning the lifeless husk to the flames, the Doctor walks away.