Supersymmetry arises naturally in string theory. (It was originally motivated by string theory.) - Nathan Seiberg

The quantum dualities, which are known as S-duality or U-duality, extend the classical T-duality and lead to a beautiful and coherent picture of stringy dualities. These exchange highly quantum situations with semiclassical backgrounds, exchange different branes, etc. As in the classical dualities, among all dual descriptions there is at most one description which is natural because it is semiclassical. All other dual descriptions are very quantum mechanical.

Whenever you work on something and try to solve one problem, and you end up helping or solving many other problems, it is a sign that what you are doing is good.

Gravity is not an option in string theory. It is a logical consequence of it!