That's an interesting question. So often we wonder and discuss 'what' we believe, but not 'why'.
I'd have to say that I believe what I believe because God revealed himself to me through scripture and his Spirit...he changed my heart, mind and how I view everything. I believe what I believe because the truth of Jesus Christ resonates in everything I see, understand and experience. There is not a facet of life where the gospel doesn't explain, change and transform.
When 1 + 1 = 2....and that 2 is the most wonderful, freeing and loving thing....why wouldn't you believe it?