The most likely reason for believing anything is knowledge of its veracity.
With enough proof, people have believed a lot of things over the years:
The slimming power of sugar
The existence of big foot
The safety of a specific type of electric current as demonstrated by this happy elephant.
Anyway, as time goes on, such notions are mostly settled as fact or fiction. Poor elephant.
Some folks here feel that the ancient belief in a round earth is one such notion, and I implore you to look through the wiki. It is enlightening.