Even when they’d got back together after Lisa, Ianto had managed to convince himself that it was just a casual thing. He and Jack were both men after all, and men have needs.
When Jack had died, the pain Ianto had felt would have surprised him if he hadn’t been too sunk in grief to really notice it, but his miraculous resurrection had swept the grief away on a tide of relief; for a while Ianto had been happy.
Now Jack had vanished, and Ianto was missing him more than he’d believed possible. He couldn’t lie to himself any longer.