Paper is made from wood, and wood is made from long fibers. Paper is made out of cellulose fiber material, and these fibers line up in a particular direction, during the manufacturing process. There is a distribution of the fibre sizes, the finer ones penetrate into the web of larger ones holding them together. Typically the manufacturing process leaves the fibers are more or less parallel. So it is easier to tear in the direction that separates fibers from neighboring fibers than in the direction that breaks fibers. Creasing paper breaks and/or separates some of the fibers, making it easier to break/separate the rest. Excessive weakening of the paper happens along the fold line because the fiber-to-fiber bonding along the fold gets loosened. Hence, once the tear is started, pulling the paper apart a little propagates the tip of the tear a little farther. Because of the loosened bonds, the pressure of tearing acts more effectively along this direction rather than getting diverted by the larger and stronger bonded fibres. So the tear follows the crease.