There is a story behind this that one day some Brahmans come to lord Shiva with a doubt on whom to worship first either Shiva or Vishnu ? So they went to ask this from Shiva and Vishnu itself, Later they have the competition between two son's Ganesha and Karthikeya of Shiva. Which has won by the Ganesha by his spirtual love towards his parents and got blessed as first worshipped God.
