One key element is his failed athletic career. Another is his military service. And his friendship with the loft - mates.
Winston's back story in New Girl has several important elements. Firstly, his athletic beginnings that left him with some unfulfilled dreams. Then there's his military time, which was a major turning point. It made him more responsible and brought out his sense of camaraderie. After that, he enters the world of the loft. His relationships with the other characters are built on his past experiences. His sense of humor, which is a big part of his character, is also influenced by his past. All these elements together make up the complex and interesting back story of Winston Bishop in New Girl.