(Okay so this is for a bit of fun. And I'm literally making it up as I go along.)
Erwin Smith and Levi Ackerman had been mated for many years. Levi was Erwins Omega, therefore Erwin felt he had to protect him. But he wasn't just protecting him from the titans.
Omegas were very rare and very prized. Alphas were respected if they mated with an Omega. This is why Erwin needed to protect Levi, he didn't want to lose him. He loved Levi very much and unlike other Alphas he has never and will never treat him like a possession. Good thing too because Levi hated being belittled.
Levi was perched on the edge of Erwins desk reading through papers. He was bored, in heat and he patience was thinner than usual. "You're frustrated. What's up?" Erwin asked.
"No shit. I mean they told me you were smart but that was astounding." Levi snapped.
"I only asked."
"The cadets have been more annoying than usual, I'm in heat and I know you don't have protection in the drawer of yours." Levi huffed.