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1) Lord Brahma created lord Kaamdev and blessed him that he can cupid
any person with his power and can wake up a will in him for Kaam. Now
lord Kaamdev thought to test that power blessed by Lord Brahma. He shot
his arrow on Lord Brahma to test his blessing. So as the blessing of
Lord Brahma was true, he got attracted for a lady, who was in relation
to his daughter.
Everyone got worried and atlast Lord Shiv
came in between both them,regenerated and brought back Lord Bramha's
conscience and cursed Lord Bramha and announced that no one in this
world will be allowed have physical relations with females who are like
their daughter,sister,mother or a higher social relation,any one till
the end of this world,committing such crime will be punished who ever it
might be,So Lord Bramha,I curse you that you will not be worshiped due
to the crime which you were going to do with a lady who is like your
daughter.
So this is one of the reason that why there is no temple of Lord Brahma and why he is not prayed by human beings.
2) According to a story in the Shiva Purana (dedicated to Lord Shiva),
at the beginning of time in Cosmos, Vishnu and Brahma approached a huge
Shiva linga and set out to find its beginning and end. Vishnu was
appointed to seek the end and Brahma the beginning. Taking the form of a
boar, Vishnu began digging downwards into the earth, while Brahma took
the form of a swan and began flying upwards. However, neither could find
His appointed destination. Vishnu, satisfied, came up to Shiva and
bowed down to him as a swarupa of Brahman. Brahma did not give up so
easily. As He was going up, he saw a ketaki flower, dear to Shiva. His
ego forced him to ask the flower to bear false witness about Brahma's
discovery of Shiva's beginning. When Brahma told his tale, Shiva, the
all-knowing, was angered by the former's ego. Shiva thus cursed him that
no being in the three worlds will worship him.