Stanley Unwin quotes:

If a publisher declines your manuscript, remember it is merely the decision of one fallible human being, and try another.

Much is written of the power of the Press, a power which may last but a day; by comparison little is heard of the power of books, which may endure for generations.

Your book may be a masterpiece but do not suggest that to the publisher because many of the most hopeless manuscripts that have come his way have probably been so described by their authors.