Born into a privileged family, he had some rather high expectations to live up to. Despite being having done well in his education as well as in the various activities he had taken part in, he never did discover his special talent. Disappointment follows him wherever he goes and felt ashamed of himself -- being sure that his family is also ashamed of him. It did end up making him very depressed for some time. One day he met another pony and became fast friends. Up to that point, he had never had a friend who had been anything less in terms of socio-economic status. They've talked for a bit and in doing so, he was beginning to comprehend a world outside of the bubble he had grew up in. Over time, this made him curious to see more and began to wonder if the answer to the meaning of his life lies outside of the life he was born into. Could his calling be found there?

 

This brought with it much conflict. For his entire life, his parents had been making many choices for them. Study this, participate in that. It hasn't worked. He thinks about running away on his own but his lack of courage has held him back. Eventually he did have the courage to confront his parents about his decision to be away from home all on his own. It wasn't easy and at first, he did get talked into calling it off but later on, his mind was made up and wasn't going to let anything stop him. He's moved out of the house.

 

He wasn't alone however. His friend accompanied him, taught him all that he needed to know to carry on his journey (this friend of his had been homeless at some point). Eventually, his friend made it clear to him that this is Sully's journey, that he cannot be an apprentice forever, that eventually he's going to have to leave the roost and fly with the power of his own wings so to speak.

 

So now Sully is traveling around Equestria hoping to discover his place in his own life.