Please someone explain this to me...

I understand the concept of lying by omission, very simple to understand, you know something and you will remain silent and withhold the information... I do get that, but yet I have very opposite feelings about that, I mean if it is a vital information that endangers someone's life or if it is an extreme important fact then I should tell, but what if it is just some minor detail, that really doesn't mean anything to anybody, or has no other purpose than hurting someone's feelings by telling, is it then still necessary to tell the truth? Who are you to have the right to mess with someone's life? Do you really have all the facts or do you just think you have them? Or do you just try to make yourself important? These are all questions that should be considered if you accuse someone of lying by omission, and a lot of times...it really is none of your business to begin with one way or another.