Why do people sometimes not notice time passing?

I'd say the main reason people lose track of time is when they're completely absorbed in something they enjoy. When you're in that state of flow, your brain just focuses on the activity itself rather than monitoring the clock. For example, when I'm designing user interfaces at work, I often look up and realize hours have passed because I was so concentrated on perfecting small details. It's the same when I'm reading a really engaging book - I'll plan to read for 30 minutes but end up spending the whole evening without noticing. That's why enjoyable activities make time seem to fly by, while boring tasks make minutes feel like hours.