That is a good short answer but prompts more questions. No leaders aspire to any position in our Church. No one seeks power and authority. When one is asked to fill a position "called" we believe the call came because Heavenly Father chose an individual and conveyed this knowledge to his earthly leaders so that they could implement it. Surprise is generally the person's first emotion to the news.
Thomas S. Monson is a living prophet on the earth today, THE living prophet. He is as much a prophet as Abraham or Moses was. He heads the Church on earth that Jesus Christ restored. This is the same as the original Church that Jesus Christ created when he was on the earth during New Testament times.
Although a religious leader now, he is an ordinary man from an ordinary background. A new biography of him has just been written and is now available for purchase.