Why do we invoke alawāt upon the Messenger ?
Sending abundant alāh upon the Messenger  is a manifestation of our love, reverence and obedience to him. He was sent as a mercy for mankind, and always remembered and worried about us.
On one occasion he  lifted his hands and while weeping, invoked: “O Allah! My ummah, my ummah!” Allah sent Jibrīl  down with the glad tidings: “Mu ammad, surely we will please you in regards to your ummah and we will not cause you grief.” (Muslim) In every alāh, he  would ask Allah to forgive us. (Ibn ibbān) He  missed us and yearned to see us.
He  once said: “I wish to see my brothers!” The Companions  asked: “O Messenger of Allah, are we not your brothers?” He  replied: “You are my Companions, but my brothers are those who have not yet come in the world. I will welcome them at the aw (blessed fountain).” (Nasā’ī)