Ignorance is bliss
Definition 1 of 1
Pro Tip 1/3Not Always Positive
This phrase suggests ignorance can be good, but it can also carry a critical tone, implying negligence or carelessness. 
He didn't prepare for the hurricane because, for him, ignorance is bliss.
Pro Tip 2/3Context Matters
Use it when being unaware is seen as advantageous, not in situations where knowledge is clearly needed. 
They didn't tell her the risks of the treatment, thinking ignorance is bliss.
Pro Tip 3/3Figurative Sense
Understand it's not praising ignorance but commenting on the temporary peace it can bring. 
Living without knowing the full extent of the problem, I suppose ignorance is bliss.