Truth alone is not the only factor in determining if we should say something or not. How we say it and the spirit in which we say it are just as important.
Here's an example: a guy sees his sister and says "You look terrible in that dress". His wife pokes him in the ribs and he says bewildered, "What? It's the truth". The wife may totally agree that his sister looks terrible in that dress but that is not why she poked him. She poked him because he was blunt and didn't consider his sister's feelings. Obviously, his sister saw something else in the mirror.
We get ourselves in trouble a lot that way. Truth can cut like a sword and most often there are ways to say things - truth - that doesn't need to cut. Sometimes we might be better to say nothing at all.
In the case above, can you picture the sister reacting with "my brother hates me"? How did she jump from his opinion about the dress to hating her. Two reasons: She did not see his statement as true and he was blunt - critical.
I've done it and probably most of you reading can think of similar situations. We say something that may be quite true but it's said far too bluntly. The other person does not see that same truth when they look in the mirror. Then they believe you hate them, especially if they are very sensitive about what you said.
If you find yourself in a situation where you are saying "What? It's the truth." - it's not the truth that got you in trouble. It's that you are seeing something different than they see when they look in the mirror. You said it too bluntly or should not have said it at all. Once the person's feelings are hurt, it's very hard to walk back what you said.