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.