Iori is a really sweet kid, and seeing him do his best (and somehow get everyone else to do their best too when he's around) warms Momo's heart every time.
But what Iori doesn't seem to know is he not only awakens Momo's senpai instincts by being his kouhai - he also makes him feel the protectiveness of an older brother, something he only gets to experience with the kids he willingly adopts after working together.
And it is a strong force, the need to make him relax and take a break because he's still not even out of high school yet. Iori is capable, and he's a natural perfectionist, and there's little he can't do if he puts his soul to it (which he does, all the time), but Momo would do anything for him to just be a kid once in a while. He wants to do his best, put his soul into that, into making Iori laugh and have a good time.
Which is why, the few times they're together out of work, the more Iori complains about things going crazy and how the way to making him happy would be to keep things in order, the more Momo just wants to throw the house out the window.
When something works, when Iori can't hold back a little chuckle, when he looks away because it is impossible to hide his smile, Momo feels a bit greedy to keep pushing it.
He should be happy with what he can get, but instead he wants to keep trying. If making jokes is what he has to do, he doesn't mind acting like a clown. If he has to give him a excuse to go out with him, such as being a senpai, he doesn't mind doing that.
Whatever he has to do to see Iori's smile is little for the happiness he shares in that moment.