I want to write down my own headcanons lol

Allen probably loves physical affection way too much. Lavi would find out first because hes always doing stuff like hugging him, ruffling his hair, arm over shoulder, etc (this is totally canon just look at how many times Lavi finds some reason to touch Allen). And so Lavi just does it way more often. Allen is 100% ok with more of it and eventually it is not a rare sight in the Order to see Allen and Lavi reading together in the lounge all cuddled up and such. And on missions when Allen has nightmares Lavi lets him sleep in his bed. And Lavi soon finds out that an effective way of making Allen not physically start fights with Kanda is to like, pet his hair.

And of course Kanda catches on and most of the time he doesn’t do anything but one day he and Allen are bickering as usual and it starts escalating but Kanda doesn’t feel like actually physically so he just pets Allens hair like Lavi does. And Allen just sits and glares because he knows what Kanda is doing dammit but its nice and Kanda like never touches him unless its a fight, so he doesn’t want to make Kanda stop and so he keeps bickering but doesn’t actually do anything else.

Lenalee notices all this too but Allen always gets a bit nervous when she does stuff like hug him because hes afraid Komui will come out of nowhere to attack him so she doesn’t really treat him any differently most of the time and just watches with amusement.