Why on Earth shouldn't I be?
Charles Darwin was (and still is) one of the most important figures in the history of science. His theory of evolution by means of Natural Selection is simple enough for a child to understand, yet powerful enough to explain pretty much everything we see in the living world. A century and a half after its publication, his theory still underpins all of the biological sciences.
As theories go, it's one of the big ones. There is grandeur in his view of life.
Darwin was also a throughly nice chap who devoted his life to science and was an unparallelled observer of the natural world. His life and work is the most well-documented of any scientist—thanks primarily to his copious correspondence, notes and publications. He was a thoroughly fascinating man.
But ever since Darwin published On the Origin of Species in 1859, he has taken a lot of flak from people who couldn't stomach his message—usually because they could not reconcile it with their fundamentalist religious beliefs. Becoming a Friend of Charles Darwin is a very good way of showing that you have no time for such nonsense.