There's a difference between discarding something you do not value vs sacrificing something you do. Sanji feels worthless and it's not like he's looking for death, but when he believes that his death has more value that his life, he is easily ready to die, because "there's nothing to lose". Zoro doesn't feel worthless, he wants to live and fight for his dream. So when he is ready to die for Luffy, this is a sacrifice. It's not that easy to explain, because in a nutshell both are ready to die for someone else's dreams, but there's more weight to it when you actually want to live versus when you believe you're not worth it.