Energy, energy and more energy when commentating. I don't care if your discussing operational psychology in regards to monetization models of F2P mobile games, be enthusiastic to make every video epic in some way.
Not easy to learn to do; however, I found first to view your time on the mic/camera as acting. However, to keep from going overboard and seeming fake (can be worse than boring), just in-vision your ideal self in a super good mood. Then learn to flip the switch when recording to produce this even when you're in a lousy mood.
It's something I am still learning and always will be, but pay attention to any of the really successful YouTubers to get an idea of how it's done correctly.