Oil breaks down with use and once it's been used in an engine for several hundred miles it now contains contaminants from the combustion cycle.
You can't go wrong by following the owners manual. But you can change more often depending on your riding style (hard/fast, economical/easy cruising, etc), your frequency (daily commuter, weekend warrior, iron butt touring, Burger show) and the environment you ride in (dusty roads, high altitude, normal, all weather).
I track our mileage and also look ahead to the trips we have planned to estimate when I should be ready to change the oil in both bikes. Both bikes say we can go 8,000 miles between changes. But my ST noticeably produces more heat that I feel on my left foot after 5,000 miles. We travel a lot and typical do 3-4 oil changes annually.
If you're riding less than 5,000-8,000 per year I recommend an annual oil change based on when the bike will be sitting the longest unused. For people that don't ride thru winter it's usually part of the "winterizing" storage prep to change oil, clean and wax, check tires, put the battery on a trickle charger, etc.
Hope this answers your question.
Terran Sin
Chief Farkle Mechanic, Valet, and Advice Columnist for Pebbles