What is an unsolicited advice? It's an advice given without being asked for in the first place. You may ask, so why is this badI only wanted to help? Well, first things firstadvice wasn't being asked or requested. Aside from the fact that you are like, 'prying' or meddling into other people's affairs, it's like putting youthe adviser, in a "one-up" position. It means that "you know" and the other party doesn't. Otherwise, you will not be dishing out advices, right?
And the other person on the unsolicited advice's receiving end, makes him in a "one-down" position, so he tends to respond negatively, though on your part, you may mean well and the help was really good. Usually, the person's response to the unsolicited advice is not even related to whether the help given is good or not. Maybe for them what they're doing is good enough based on their own standards or through experience, so they don't need any help at all.
You have to realize that not all good intentions always equal to better relationships or even good communication. Though you might mean well, and you're offering your help, and you may reason that you're just being proactive and taking the initiative. And don't even think that you get hurt or miffed because the other party didn't appreciate your effort in giving suggestions. It doesn't always result nicely. Offering unsolicited advice is not really bad, it's just unnecessary, for me that is.
Next time, it's always at the best of everyone if people like to hear a suggestion or if they come to you asking for help, before simply surprise them with your "wisdom and sound judgment" just like that which may not be pleasant to them at all. In this way, this gives them time and space to share enough information with you.