In Islam, the Allah is the one who is to be given the highest respect and worshipped. Next, is the Messenger of Allah, more than anyone else.
It is because Allah has commanded us to respect Prophet Muhammad (SAW). He has told us that the Messenger (S) was on a dignified status of character. Therefore, all humans must send blessings upon the Prophet (SAW) just as Allah and his angels send blessings upon him. He has also ordered us to send blessings upon him whenever his name is mentioned.