One of the things I love most about being a doctor is knowing things. One of the most important things one learns in medical training is how to "interpret the literature" — how to find studies that apply to a clinical question and then how to tell if a given study is any good (a surprising number of them aren't, or are only useful in a very limited way).
I also love sharing my knowledge. I've never been annoyed by friends and family members who ask me medical questions. Since I'm an internist, a lot of the things they ask are the same things I manage every day with my patients. And I'm very comfortable admitting when I don't know something.
But I don't want to become little Dr. Know-It-All. Sometimes when roaming around in the blogosphere, I come across descriptions of medical issues that seem either wildly inaccurate or possibly mismanaged, and I have to sit on my hands to not comment. For instance, I came across a mention of someone being treated for years with intramuscular antibiotics for "chronic Lyme disease." Although I know that this is essentially malpractice, it's none of my business, and I'm sure the comments of an anonymous stranger aren't going to change things anyway. But I hate thinking that someone else reading it will be misled.
There are a lot of murky areas in medical knowledge, and I enjoy reading educated debates about these issues. So much of what we once "knew" to be true has turned out to be wrong, wrong, wrong. Humility is a critical attribute for a clinician. But so is confidence, and it's a difficult balance to strike sometimes.