Bolin was ready and rearing to accept the invitation. As soon as the door opened he all but flung himself inside the relative safety of his brother's room- which, if he stopped to think about it, was just like his room, and probably not any safer.

But hey, power in numbers. Right? Sure. Let's roll with that.

Once he got situated- i.e., once he hopped into Mako's bed and made a cocoon out of Mako's bolster (with only his face visible) -he smiled a bright smile at his brother.

"Hi. I thought you might be lonely. I'm just considerate like that."