too much of a good thing

Too large an amount of a beneficial or useful thing or activity can be harmful or excessive. For example, The indoor decorations are fine but the outdoor Santa, sled, reindeer, gnomes—it's just too much of a good thing. Expressed in slightly different form even earlier, Shakespeare used this precise wording in praise of moderation in As You Like It (4:1): “Can one desire too much of a good thing?”