Answer:
Sine is equal to opposite over hypotenuse in right triangles. Let's say we have a 3-4-5 right triangle, the side opposite the 90° angle is 5, and 5 is also the hypotenuse. 5/5 = 1
Therefore, in all right triangles, the side opposite the 90° angle is equal to the hypotenuse, and anything over itself is equal to one.
(Not sure if this is a mathematically correct way to explain it, but it makes the most sense to me.)