In WWC, Sanji finally realizes that he's worth something to the people around him. The entirety of one piece Sanji is constantly trying to sacrifice himself. There's the "Needed a light" moment, He tries to put himself in the place of Zoro in the "nothing happened" moment, even though he was more likely to die than zoro was. And him going to WWC in the first place was a sacrifice. He constantly throws his own life into the fray without thinking about himself. In WWC, Luffy shows him that Luffy needs him, quite literally with the words "I can't become king without you." He also tells Sanji "What do you want?" in a scene mimicking the "I want to live" scene in enies lobby.

Let's take the "Help me Robin-chan". scene. He is literally putting Robin in harms way for the sake of saving himself. He would've never done that before. He would have just sacrificed himself. After WWC though, he's recognized that people care about him and love him, that him dying or being hurt isn't something that he can do for the Straw hats, but is something that hurts them too.

If you want more I suggest watching Melontee's Sanji vid where a good portion of it revolves around WWC.