The two sides of 2 and 3 combine to form 2+3 = 5. Imagine laying two sticks out of length 2 and 3. They would form a straight line and not be long enough to bend at the hinge to allow for the other side (size 5) to fit anywhere. A triangle does not form. Instead its simply a straight line. For a triangle to form, any two sides must add up to some value larger than the third side.
