1
general::
Phrase(s): too much of a good thing
more of a thing than is good or useful. • I usually take short vacations. I can’t stand too much of a good thing. • Too much of a good thing can make you sick, especially if the good thing is chocolate.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
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?"
American Heritage Idioms