you're not dumb anon! I just like to take any connection that tethers them together and run with it. I love Shinobun too!!

"Everyone here is appreciating the things I like. It makes me feel like I belong here." (Shinobu, to Midori and Nazuna, Chuunibyou)

Ninja Senpai
In Scrolls of the Elements, Shinobu brings up a potential gig for a joint live performance between Akatsuki and Ryuseitai. Chiaki leads the charge in convincing Keito to take it on and make it into a proposal. Keito does so and praises Shinobu for running the Ninja Association on his own.

“It seems you are sponsoring the Ninja Association or such, aren’t you? You are the master of your own house. Be more resolute. Isara was worried, you know. Although club activities are gatherings for hobbies, strive at it without reserve.” (Keito, Scroll of the Elements)

After the events of Scrolls, Keito grows interested in Shinobu as (a) a member of Broadcasting Committee (centralized information) and (b) a member of the Student Council. To keep tabs on Mao, Keito employs him as a secret emissary.

“At first, I asked Sengoku if he could do it, considering I’ve been getting him to keep me regularly updated on how the Student Council is doing, as I tend to worry about Isara as the president.” (Keito, Submarine)

In Submarine, Keito asks Shinobu to fulfill the same duty. However, Shinobu refers him to Chiaki instead: Chiaki's secret order makes it easier to be discreet.

"Alright, so… We began sharing information with Akatsuki after forming a common front. At first, I was the one who was asked to be the mediator, but this time around, Morisawa-dono was much more convenient for the role, so I requested that he do it in my stead.”
”Yup! I was Ninja-man, who went around Okinawa gathering information and passing it on to the right people!”
“Ninja-man! That’s so cool~” (Shinobu and Chiaki, Submarine)

“Sengoku seems to be happy to have a chance to act like a ninja, too! Thanks for playing with my junior, Hasumi.”
“I’m not playing with him… Though I do think I should reward Sengoku sooner or later for acting as my subordinate.”
“I recall him saying he’s simply returning the favor you did for him in Scroll of the Elements, so don’t worry about it.” (Chiaki and Keito, Submarine)

And Shinobu’s not the only lonely one of the three.

Midori: “Ahh, dammit…! You just had to mention you were bored…!”
Madara: “Ho ho. Chiaki-san, you said that? You couldn’t get yourself to mention it before. What made you change your mind?”
Chiaki: “Ah, no. I just happened to see them yesterday, that’s all. I ended up mentioning how bored I was, and Sengoku invited me to hang out. He also had free time for similar reasons as mine, so he was more than happy to spend time together. Hehe, looks like I was holding back more than necessary.” (Buddy)

It’s really cute that they both indulge him, and I just find it special Shinobu has senpai who look after him and empower his interests. Post-Submarine, I like to imagine Chiaki trails after them like "how are you doing, fellow ninja? (:" [seeking connection with his kouhai and companionship with the two of them].

"I've trained so much, but a lot of the jutsus still ended in failure... It's kind of depressing, but at least the Jutsu of Summoning I tried in the past was a success... "May I make friends", I said." (Shinobu, Chuunibyou)

(And if you're curious, here are some Dokisuta relationship charts)
(Shinobu's chart)
Keito -> Shinobu | Tasks him with keeping him up-to-date about the academy
Shinobu -> Keito | Easy to get along with
(Keito's chart)
Keito -> Shinobu | Cute junior
Shinobu -> Keito | Respects

Chiaki -> Shinobu | Reliable and energetic
Shinobu -> Chiaki | Respects and trusts him

Sources
Buddy (TL | 310mc)
Chuunibyou (TL | Bakemonoremy)
Scroll of Elements (TL | Looks after rainbows)
Submarine (TL | 310mc, verdantgrove)

Shinobu Dokisuta Relationship Chart | (TL | 310mc)
Keito Dokisuta Relationship Chart | (TL | maplelotus)* TL is a generous way to say it, ahaha, it was two lines.