in·a·ni·tion, noun; lack of mental or spiritual vigor and enthusiasm; exhaustion caused by lack of nourishment. (In which Luffy's metabolism is faster than his doctor understands.)

Sanji didn't wonder why he gave himself the challenge. It didn't matter why. ‘Why’ was probably covered by one-upping the bloody swordsman, and maybe there were other reasons - stranger reasons - but that was the only one that really counted, right? It was decided. He was going to get Zoro to appreciate - no, love Sanji's food. If Sanji couldn't do it, then he wasn't a true sea-cook after all.