Santiago's inborn talent was to follow his senses and the omens of his personal legend; so, Mstykmshy certainly has answered correctly. Another aspect of Santiago choosing to be a shepherd rather than a priest was the fact that shepherds travel. Santiago was always interested in traveling to see all of Spain; little did he know that part of his personal legend was to travel half of the African continent as well. Had Santiago become a priest, he would have had to follow specific daily rules that wouldn't allow him to travel or to be free to think for himself. He wouldn't have been able to explore his inner sense to talk to nature, the universe, or himself as he discovered on his journeys. When mentioning the word "priest" one might think of stiff pants and shirt with a high white collar practically choking the wearer. This is not Santiago. Santiago's character and personality demanded freedom to explore the world, himself, and even the universe.
Santiago followed his own dream instead of simply doing what his parents wanted him to do to. This relates to the idea of following your own "Personal Legend" which the story revolves around. Santiago became a shepard because he wanted to travel and being a shepard was the cheapest and easiest way to do this. Being a shepard was a part of his personal legend.