The Metaphysical Elements of Ethics by Immanuel Kant Chapter 8 Page 6

obtain help from them in case of necessity), we therefore make ourselves an end for others; and this maxim can never be obligatory except by having the specific character of a universal law, and consequently by means of a will that we should also make others our ends. Hence the happiness of others is an end that is also a duty.

I am only bound then to sacrifice to others a part of my welfare without hope of recompense: because it is my duty, and it is impossible to assign definite limits how far that may go. Much depends on what would be the true want of each according to his own feelings, and it must be left to each to determine this for himself. For that one should sacrifice his own happiness, his true wants, in order to promote that of others, would be a self-contradictory maxim if made a universal law.