No one ever wants to hear the words, “I told you so.” Besides they fact that saying “I told you so” is going to make you wildly unpopular, there are also some other very good reasons not to utter these words to those that you love and have relationships with.
Saying “I told you so” is not going to make the person that you are talking to hear you better. It is not going to make them want to take your advice better from now on. In fact, it may make them listen to you even less than they do now. It may make them not want to come to you for advice at all. It could turn them off from telling you anything personal about their life. It will not bring others closer to you; it will more than likely repel them from you.
It is not helpful. We can all look back and see things that we could have done better of differently but hearing someone say, “I told you so” is of no benefit. Think about how you feel when you have had this said to you. Chances are, it didn’t make you think how helpful that phrase was.
It can cause anger, bitterness and resentment between you and the person you are saying this to. There is no relationship where it is beneficial to say this, even if you warned the person of what could happen and that proved to be true. It is really a disrespectful and unkind phrase.
A better approach is to be there for the person you warned and help them as they make their way through whatever mistake it is that they made.
For more information on things is it better not to say, you might find this article helpful by Lifehack.