It's not just the souls of monsters that kill each other for all eternity, but the world itself. Purgatory spills out it's guts, devours them, and regenerates, parts growing back different than they were before.
Sometimes Dean falls asleep in a forest, and wakes up on a rocky plateau. After about a month, the ever-changing landscape doesn't unnerve him as much, though he's still vaguely worried about waking up at the bottom of a lake.
Cas assures him that that won't happen.
Castiel knows this place, and it knows him. Which means that Purgatory targets his weaknesses, drains him and wears him down. But it also means that Castiel can see past the physical landscape. He can see the paths radiating from the centre of Purgatory, the path to the gate that will take him and Dean home.