First, it's patronizing. The natural reaction of the one being advised is to feel indignant: how arrogant of someone to think that they know better than me how to run my own life! And so, whether the advice is good or not, the resentment of being talked down to is often enough to ensure that the advice will be ignored. Which isn't so bad, really, because—

Second, the advice is usually wrong. People don't know how much they don't know, but they think they know, and think they can help others by telling them what they think they know. It's tempting to think that once you've been told about this tendency, you can correct for it, and give genuinely good advice that takes into account what you don't know, but you're probably mistaken about that, because—

Third, telling people things mostly doesn't work. Natural language is the only means we have to communicate thoughts with each other, but it doesn't necessarily work very well on an absolute scale. You can try to sum over what you've experienced and package it in a few natural language sentences of advice, but the words are going to be interpreted in the context of the listener's experiences, not the context in which you generated them; it takes years of study and practice to transform verbal lessons into usable, actionable knowledge. Get what I'm saying? That's right: probably not.