how right is it to let someone know the truth? especially if they’re going to be hurt by it?

on every TV show i see now, there’s someone saying the line “i think you should know…” and proceeds to tell them something which changes things around them. whether it be a girls best male friend telling her he loves her right before her wedding wows, or someone you invited to the dance telling you they lied to you, it’s all done on the pretense that the truth should be known. 

i’ve grown up with the motto of keep things to yourself if you believe them to be detrimental to someone else, especially if they have to concern feelings. however, today i’m in a dilemma.

so i ask anyone whose reading this…

“is it wise to tell someone the truth that would, at best, ruin their day, and at worst permanently alter their state of mind, especially when they haven’t asked for it?”