I don't have a lot of rules that I apply across-the-board. But "Keep your advice to yourself" (unless directly asked for it) is one of them. Why? I have come to know, through my work with many, many different people over the years, that we are all unique in so many respects - our inherited temperaments, our learning styles, our histories - that what holds true for one person may be completely false, or at least inaccurate, for another. So, with just a few exceptions (which I'll discuss below), the advice we give has a good chance of being inappropriate, or just plain wrong.
Another reason to keep your advice to yourself: people generally don't want to hear it, and don't appreciate it. Jacob Glass*, the teacher who has had the greatest influence on me, states that to understand human behavior you need to keep in mind that what everybody really wants deep down is to be adored. The cry that almost all little children utter when they're doing something they're proud of: "Mommy, mommy, look at me" expresses this desire, which I believe stays with us throughout our entire life (just read some of the posts on Facebook). Giving someone unsolicited advice suggests that there's something wrong with them or with what they're doing - hardly an adoring stance.
You'll notice that I do allow for the exception "unless directly asked for it". Directly. Knowingly or unknowingly, we sometimes maneuver people into getting them to ask for our advice. Just yesterday I was working with a client in whose sessions advice-giving has been a prime topic. She told me that she'd recently had a long conversation with her brother, who is facing numerous difficulties, and she proudly exclaimed that she'd resisted the temptation to give him advice. She then went on to tell me that she'd asked whether it might be helpful for him to hear about her experience with a couple of situations similar to those he was facing, and then shared some possible options with him. She believed that by not telling him what to do (but instead asking if he'd be interested in her experience and then suggesting several possibilities) meant that she was not giving advice. But she was. Multiple choice advice is still advice.
So when might it be OK to advise someone on something? If you are talking to someone who works for you and you're paying them (in money or some other kind of remuneration) you are certainly entitled to give that person advice - but only on the work for which they're being paid. Another exception would be in talking to a pre-adolescent child. PERHAPS in a healthy, long-standing romantic partnership or marriage. But that's about it.
Well, suppose you have a friend, relative or colleague who you absolutely know is struggling with an issue about which you feel you have valuable expertise? Perhaps he / she says "I just don't know what to do?" Sounds like the perfect opportunity to jump in with a point-of-view. But saying "I just don't know what to do" is different from asking "what should I do?" And "what should I do?" is different from "what would you do?" The latter is a direct request for advice. "What should I do" sounds like it, and it might be fine for you to advise that person, but it will probably be more valuable for you to encourage a discussion of what options have been considered, asking about the thoughts and feelings associated with the dilemma. Try to help the people you care for reach the conclusion that's right for them, rather than what's right for you.