Why does the pronoun you break all the rules?

Imagine you see one jovial-looking chap.  You might say:
He is happy.   (singular, 3rd person)
And if there are two, you might even say:
They are happy.  (plural, 3rd person)

When you are talking to them, you would say:
You are happy.        (singular, 2nd person)
You are happy.        (plural, 2nd person)
Why not you is?

If you were back in the 1400's, you might have said:
Thou art happy.         (singular, subject, 2nd person)
Ye be happy.              (plural, subject, 2nd person)
I love thee.                (singular, object, 2nd person)
I love you.                  (plural, object, 2nd person)

So, you was actually for multiple objects.  As the language changed, it became the formal form for all of the above cases and then eventually consumed them all.
These days, we do not use thou, thee or ye.  And you has become the pronoun for both subject and object positions.

It also holds the title for addressing both one person and crowds.  Many languages have two seperate words and colloquially, we fill this missing lexis with you guys or y'all for the plural form to make it clear that we are speaking to everyone not just one person.