its cuz Yuri told him so. like ch 7 or whenever they met. idk why he believed him but it was some stuff about how busy al is and havit a psrtner like zen is a burden, how he's asking too much of him and he should try taking on another dom to lighten the load off zen. im tryna give him a smidge of grace cuz hes in another universe and actually doesnt know if thats true or not... but he should have just asked al if it was and then moved on. But i also think Yuri kind of commanded him to not tell Al so that probably caused some of the internal conflict.
It’s also because the doctor did tell him that he needs to play often in order to get rid of the sensitivity and since Yuri told him Al is really busy and he’s only be burdening him, he just believed he would indeed be a burden to Al instead of just directly asking him. Also, he did tell Yuri he felt like he was being coerced into doing the plays which he was because Yuri knew just how sensitive Zen is, even going as far as calling him a “special case”
Zen is too dumb to just let Yuri do that, and Yuri's a prick who can't mind his own business. They know they are mot compatible so they should've stop. Zen is fcking dense to not see how well Al is during and after they play. I'm guessing there'll be a proper communication or worse Al will go ballistic that even Zen won't be able to handle the glare or the misunderstanding wouldn't be resolved next chapter because Al needs space and Zen's would blame fvck out of himself for being too dumb.