Since Buchanan is the only President of the United States to never marry, there has of late been abundant speculation about his sexual orientation. There is no evidence to support any conclusion in that regard, and in any event, there is no indication that this would have affected his judgement as President.
Buchanan was brilliant, and had a distinguished career in government before he became President. He served five terms in the House of Representatives and ten years in the Senate. He also served as Ambassador to Russia and later to Great Britain under Franklin Pierce. There is some argument that because he was out of the country, he had managed to avoid the intense sectional debate which had roiled the nation, and was therefore an obvious choice for the Democratic nomination.
As President, Buchanan failed to realize the depth of sectional controversy. As President-elect, he had hoped the Supreme Court would settle the controversy over slavery in the territories; but was proved seriously wrong when the court issued its decision in Dred Scot vs. Sanford within days of his inauguration.
Congress remained at a stalemate during his administration, and like Pierce and Filmore before him, any decision he made angered a substantial element in his own party. During the interregnum from Lincoln's election to his inauguration, South Carolina and other states announced their secession from the union. Buchanan denied they had the right to do so; but at the same time held that the Federal Government did not have the right to stop them. After leaving office, he retired from politics completely and lived only another seven years.
If you are talking about aspects of Buchanan's life that were relevant to him becoming president, then the most important aspect of his life is that he was not in the United States from 1853 to 1856. This meant that he had not been involved in the controversy over the Kansas-Nebraska Act.
The controversy over that act was what ensured that other, more charismatic, men did not get the Democratic nomination for president. Stephen Douglas, for example, was much more dynamic and well-known. But he had been the one in the thick of the Kansas-Nebraska Act controversy and so there were too many hard feelings against him.
Buchanan, on the other hand, had been in politics a long time but was not controversial. Being out of the country, he had not had to take sides on the Kansas-Nebraska Act. He was from the North but had voted with the South on various issues in his career. Because of this, people in both the North and the South could accept him.
These are the aspects of his life that were most relevant to him becoming president.