Why are we very keen to remove sulfur and nitrogen from the oil?

We do Desulfurization to remove sulfur becase (1) Sulfur is poisonous for catalyst (2) it produce bad odour & SO2 combustion is harmful for environment.

we proceed to the desulfurization of oils because sulfur compounds - sulfur- are harmful catalysts, to their unpleasant odor, production of sulfide oxide gas during combustion.

a result of some refining process (eg catalytic cracking) a significant fraction of nitrogen may be present in light cuts and deteriorating quality : storage instability , brown color, formation of rubbers...