Here's why:
* Subject-verb agreement: The subject of a sentence should be in the nominative case. "You and I" are the nominative forms of the pronouns.
* Standard usage: "You and I" is the standard way to refer to yourself and another person in the subject position.
Examples:
Incorrect: Me and you should go to the store.
Correct: You and I should go to the store.
Incorrect: Me and you are going to the party.
Correct: You and I are going to the party.
However, "me and you" is acceptable in informal speech and when used as an object.
Example:
"The teacher gave me and you extra credit." (Here, "me and you" are the objects of the verb "gave.")