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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.