Yea, at work I had to go through the explanation phase too. "Why aren't you eating anything?", "Is that all your'e going to eat?", etc. Then, when I adding weight, I got to explain why I was eating a half dozen eggs for an afternoon snack, why whole milk is good for you, why it is ok to eat carbs, etc. As Soldier points out, those conversations almost always go to dieting, and they say "I could never do that", they tell me what I'm doing isn't healthy, or they explain their diet (which is clearly not working).
Yet they marvel when I tell them that I LOST 2 pounds while on vacation in Florida (a coworker was complaining about having gained 7 pounds on a week's vacation). That's b/c even though I was eating "junk" on my vacation (plenty of fish, fried food and beer), I still wasn't getting in the total calories that I was used to. So even though I wasn't exercising at all while on vacation, I still lost weight.
I've always been polite when asked about it, but it does get tiresome. Now, I try to avoid the subject entirely. The bottom line for me is that most of my coworkers, particularly those that want to lose weight or get in better shape, lack the will power, motivation, and drive necessary to achieve their goals....but I don't tell them that, instead I say "you get out of it what you put into it."
"You're right, you can't do this. That is why you're fat and I'm awesome."--LOL. I want that on a T-shirt.