Benjamin was Jacob's baby boy; the second son of Rachel. Benjamin was Jacob's last son to make the trek to Egypt. His only full blooded brother, Joseph, even treated him as special.
When the land of Canaan was divvied up, Benjamin received a portion right up against Judah's. This was significant. Although there was eventually a civil war which featured Benjamin against Judah (primarily); and although Benjamin was almost completely eradicated in that conflict, Judah and Benjamin became very close indeed. Over 30 times in the Old Testament these 2 tribes are mentioned together. Looking at this from a very practical standpoint, the fact that the first 2 kings of Israel came from Benjamin and Judah respectively must have had something to do with their bond, especially since David had such deep connections with Saul and more particularly with prince Jonathan.