Moving is one of the basic pleasures of life, and speed usually increases it. If you are using the bike for transportation, and it is 1.5 times faster than before, you can get to over twice as many places on a grid in the same time. If you are using it for pleasure, a light bike has a better feel going over bumps. When the bike is barely taxing your power to move itself, the responsiveness encourages your efforts. A faster bike is usually a more expensive bike, and that investment can help with motivation to ride. This all has to be balanced against the risk of theft, though.