Is repentance for a harmful sin—such as masturbation—only for the sake of Allah because He has forbidden it, so that it may be for the sake of Allah, or is it due to its physical harm, and does health, as one of the objectives of Sharia, have a connection to this?

True repentance is solely for the sake of Allah Almighty, with the intention of drawing closer to Him and obeying His command. Repentance merely to ward off harm is flawed. The prohibition of masturbation aligns with the objectives of Sharia in preserving one's self and health. Whoever abandons it with the intention of achieving this objective will be rewarded for it, and this does not invalidate their repentance, for it falls within the scope of obeying Allah's command.