Because I don't want to tell my boss about my personal life but I feel like I have to, since he asked me direct question about it. Better to start with some simple but functionally relevant topics, if you want to transition. Or just say, slowly and calmly, "it has been a crazy day today, huh? Let's take a moment to catch our breath and take stock of what's going on.
Small talk doesn't have to be about one's personal life. Most people are happy to talk about their kids, etc, but questions like "What's up?" or "How are you?" can be answered in all sorts of ways.
If you're uncomfortable talking about something personal, I'd suggest you just dodge the question. "Oh, it's fine. What's really on my mind is [work thing]." Unless your boss is totally clueless, they'll pick up on that.
