OK, here we go... a lot of Cats have stripes. Remember the cat was a desert animal first, they why the instinctually use a litter box. Deserts and plains regions had tall grass and the stripes help hide them in the grass during a hunt. Most dogs are mottled. Remember the dog was a woodland animal mostly. stripes wre not the best camoflauge in that environment. Mottled was best in order to blend with the scattered light on the floor of the forest and the leaves in the background. These days we tend to forget this because they have been out of this environment for so long and have been modified from thier companionship to us. Overall it was just an adaptation to thier surroundings that gave them this appearance.