Mike was gonna become a brick in the wall since S1, and it was always connected to him conforming in his relationship with El.
We NOW know from S5:
At the end of the song, a man shouts:
If you don’t eat your meat, you can’t have any pudding!
This is obviously a metaphor for conforming: enduring your unpleasant duties first, do what's enforced and expected of you (eating meat, the proper food), only then being allowed something you actually enjoy: freedom and joy (the dessert).
In the finale of S1, Lucas and Dustin are quite excited to find loads of pudding:
Mike doesn't seem so interested:
El seems intrigued but doesn't know what pudding is (this is so sad: she doesn't know joy, only her duties):
Mike explains:
El also wants to eat Eggos, but Mike says no, eat real food:
Eat your meat!
This is where ST first uses this metaphor and it's right before Mike kisses El for the first time. I don't need to explain that that kiss does not look enjoyable, that Mike surprises her with it and she asks him if he's like her brother right before.
Then again in S3, during Mike's first attempted love confession to El, they sit in front of this sign:
Mike tries again to perform his expected obligation to reconcile with her after the break up to become her boyfriend again, and isn't successful this time or any other time really telling El that he loves her.
So this metaphor has been connected to Mike from S1 and specifically to Mike and El and the quintessential hallmarks of romance (first kiss and first love confession), being an unpleasant obligation enforced on them by an oppressive and authoritarian system.
