Charlotte's web

This is a thematic question. I always thought that friendship is the centre of this novel. 

Friendship is the foundation of this novel. Without Fern’s love for Wilbur he would have been killed as a runt and without Charlotte’s love he would be served up on the Christmas dinner table. Despite the fact that some view this novel as pessimistic, Wilbur is ultimately saved (twice) by love -- a fundamentally optimistic message. Although this is not a typical fairytale romance where boy and girl meet and fall in love, the love here is equally powerful.

As Charlotte says, "what's a life, anyway? We're born, we live a little while, we die. A spider's life can't help being something of a mess, with all this trapping and eating flies. By helping you, perhaps I was trying to life up my life a trifle. Heaven knows anyone's life can stand a little of that." Although Charlotte perishes in the end, her life is given meaning by helping Wilbur and the effort she goes to in order to save him makes her feel happy that her life has had purpose.