Someone who is unfailingly cheerful — no matter what — can be described as pollyannaish. If your friend tells you some terribly bad news, it would be pollyannaish to say, "Yes, but it's such a beautiful day!"
While the adjective pollyannaish describes an optimistic outlook and a determined cheeriness, it also implies that this attitude is taken too far. When you put a positive spin on everything, even things that call for sadness or discouragement, you're being pollyannaish. The word comes from a 1913 children's book by Eleanor H. Porter, Pollyanna, about a young girl who tries to find something positive in every situation — a trick she calls "the Glad Game."