Hooks are pretty sharp when they leave the hook factory, but they get knocked around in your fly box, they pick up minute amounts of rust (look at some of the flies in your box with a hand lens), and they get nicked on rocks all the time. There is a huge difference between a hook that is sharp and a hook that is really sharp—a really sharp hook is so sticky that many fish hook themselves, yet few people realize it because so few sharpen their flies.
There’s no voodoo to sharpening a hook. Draw a hook sharpener against the point of your hook a few times (parallel to the shank) on the bottom, and then take a couple of quick strokes to each side of the hook. Check it by drawing the fly across your thumbnail at a 45-degree angle. If it sticks into your thumbnail instead of sliding across it’s sharp enough.
You can use a fine emery board, a fine ceramic or Arkansas stone knife sharpener, a fine diamond file, or in a pinch a piece of fine emery paper. I really like the ones we sell, which are adhesive-backed boride hones that I stick to my snips. You can also stick one on a fly box, net handle, and any other handy surface. At $6.95 for two of them, it’s the best fishing insurance you can buy.