Disclaimer: I don't own Merlin.
Seeing a dagger being sent toward your only son is one of the most frightening things a father can go through. He was helpless. He could not do anything except watch as the dagger came closer and closer. With each passing second, the terror he felt grew inside of him.
Right as the dagger was about to strike, his son was suddenly pushed to the ground. The dagger that was meant to pierce the heart of his son buried itself in the back of a chair. Looking at the ground, Uther saw his son lying on the ground along with another person. Gratitude for the unknown man filled him. If it wasn't for the skinny, pale young man, his son would be dead.
This feeling of gratitude was what prompted him to reward the young man with a position in the royal household. It was the least he could do for saving the life of his only son and heir.
I'll try and update Chasing the Creature and The Day After tomorrow.