I think, for me, it comes from a deep desire and need to be "noticed" and when I am observing someone or listening to someone and there is an opportunity for me to start "fixing" I feel this compulsive urge to do so.
I've started telling myself: just STFU. STFU, let them talk, and if after talking they open the door for feedback or thoughts or observations share them, otherwise they only want someone to acknowledge what's going on for them.
Unsolicited advice is also really vicious the more I've thought about it, you're robbing someone of the opportunity to figure things out for themselves, even if you've figured it out yourself or you have a great way of doing things, most people will and should figure things out on their own (and those that want to improve will open themselves up to feedback).