"You're what you eat" - why does that sound wrong?

[Revtim]

@Patty_O_Furniture

I noticed this on a refrigerator magnet today and had a knee-jerk chuckle response at the perceived grammar error. Then I thought about it and couldn’t figure out why it was an error. “You are what you eat” is ok, so why does “You’re what you eat” hurt my ears?

There are plenty of situations where “you’re” can be used to start off a sentence:

You’re right!
You’re late.
You’re fat/bald/ugly.

But in all of these cases the stress seems to be on adjective. In the case of “You are what you eat”, the stress is shifted to the linking verb (are), I suppose to stress the equality of (you = what you eat).

Is that why it sounds wrong to make “You are” into a contraction in this case, or is there a grammar rule I’m forgetting about? Are there any other cases where we ought not to use contractions even though it might be grammatically correct to do so?

Did you grow up watching Saturday morning cartoons in the 70s and maybe the 80s?

There was a frequently-shown PSA type cartoon where a character would sing:

You are what you eat!
From your head down to your feet!

Perhaps the phrase became embedded in your mind with “You are” from hearing that song over and over, and anything slightly different sounds wrong, even when grammatically correct.