Too much of a good thing
Definition 1 of 1
Pro Tip 1/3Negative Effects
The phrase suggests that even positive things can become negative with excess. 
She realized that buying too many shoes was too much of a good thing when her closet overflowed.
Pro Tip 2/3Common Collocations
It's often used with 'can be harmful' or 'is never good' to emphasize the potential danger. 
Studying is important, but remember that too much of a good thing can be harmful for your health.
Pro Tip 3/3Situational Use
Used when warning someone about overindulgence in activities they enjoy. 
Playing video games all night? Too much of a good thing, isn't it?