Dr Ambedkar in The Buddha and his Dhamma, underlines how, contrary to being pessimistic, Buddhism is a religion of a dynamic middle path. He asks in the Introduction (page 19):
If life is sorrow, death is sorrow and rebirth is sorrow, then there is an end of everything. Neither religion nor philosophy can help a man to achieve happiness in the world. If there is no escape from sorrow, then what can religion do, what can Buddha do to relieve man from such sorrow which is ever there in birth itself? The four Aryan Truths are a great stumbling block in the way of non-Buddhists accepting the gospel of Buddhism. For the four Aryan Truths deny hope to man. The four Aryan Truths make the gospel of the Buddha a gospel of pessimism. Do they form part of the original gospel or are they a later accretion by the monks ?
He addresses this question in the later part of the book (page 428):