What I really appreciate about Charlotte's Web, is the honest and sincere friendship to really, the end, and again continuing on from there.
In Charlotte's Web, the friendship overcomes many obstacles, and one way or the other, they conquer it.
An example of what i am trying to say is, when Avery was trying to capture Charlotte and put her in a box, (and then after that God knows what he was going to do...) Wilbur was going hysterical and made Avery fall over, and there for, breaking the rotten goose egg and make then run off. Wilbur in a way DID save Charlotte's life.
Another example of how friendship is a very big thing in this book, is when Charlotte makes her web in a different pattern just to show to the world how he's a very extraordinary pig, in every single way. So in a way she was sacrificing her web to save her best friend.
This book has many more examples, but these (to me) are the most essential to the book plot.