Eren was a brat. He knew it. Levi knew it, and all the squad knew it. Everyone knew that Eren was a brat. What they failed to know however was the fact that he was Levi's brat.
Levi noticed things about Eren just like Eren noticed things about Levi.
Like when Levi wanted sex Eren was the first to know. It was easy for him to read the older man. The way he walked, talked, heck even the way he sat!
On normal days Levi would walk tall and strong. Not missing a step. He looked to people with bored uncaring eyes. No one would dare talk to him.
On days he wanted sex he wasn't as forceful. He had a slight slump to his walk. Eren being Eren he noticed this. The slight slump would get more and more noticeable as the days went by. In that case Eren would always give in only to have his captains strong walk back again the next day.
There was yet another thing that Levi did when he wanted sex. And that was the way he talked. When he talked his voice would get a pitch lower. One that Only Eren would notice. When he would talk to the boy and command him to do something Eren knew that it was just a command to know what he wanted. Levi's voice would go back the day after. Sometimes Erwin would notice this but never ask about it.
And last? The way he sat. On most days Levi crossed his legs when he sat. On days he wanted sex he would keep them slightly open. Even leaning over a bit to show Eren what he wanted. At this point Eren knew what he wanted and that was a hard core night. He hardly ever sat like that. And when he did ropes were used that night.
Eren never knew what would happen if he were to ever let Levi go for longer than a day. He often wondered what would happen. So he came up with a busy week. To make Levi wait. Between training with Mikasa, testing with Hanji, and hanging out with Armin he had a busy week planned. Ready to leave Levi hanging.
Day one was fine. Like always Levi was more into his work. After all it was the day after. Day two went by smooth with very few signs. Things Eren would miss on a normal day. Day three Levi was hinting more and more every time he even got to see Eren. By day four Levi was hinting even going as far as to make Eren bring him tea when he wanted it. Day five Eren made sure to not be a second alone with the older man. Levi was not happy. Day six passed rather fast with Levi being pissed at everyone and everything.
When day seven came Eren was forced out of the lab by Levi. He was being dragged by his hair to the older boys room. Eren left out a few ow's but it only made things worse for him. He was tossed to the floor and Levi slammed the door jumping on the poor boy. Eren didn't have a say in the matter but after a week he really didn't mind being jumped.
So to end his research. Never let Levi go without sex for a week. Cause by day eight you're the one in pain. Not him. Well at least you get pampered. That's an upside!