Erwin is too pleased with himself for getting away with this to notice how Levi is responding, beyond the fact that he's not actively trying to get away. That means he doesn't really mind being picked up, so Erwin is going to milk this for all it's worth.

"Let me have my fun." Which is, apparently, carrying Levi out of the kitchen and towards their bedroom, pushing doors open with one foot as he goes. "I haven't been able to do this before."

Back when he could've, he didn't, and then he was allowed, but didn't have the two arms necessary. Now he's both allowed and equipped with two arms, so he's going to take advantage where and when he can. Knowing that he can do this, that the robotic arm is strong enough to lift Levi, also helps ease a certain constant, aching worry in Erwin's chest: what if something happened to them, either in the Labyrinth or when something was attacking, and he wasn't able to carry an injured Levi to safety? He'd end up staying behind with him, he knows he would, but being able to get them both out of danger is the clearly preferable option. Knowing it's a possibility might make some of the nightmares go away.

Once in their bedroom, Erwin lays Levi down on the bed, then hovers over him, not straightening up immediately. "Would you like me to take care of your pants?"