One was political. In the mid-1960s, when it seemed that everyone in power and fame was in favor of the Vietnam War, or certainly condemning of anyone who was beginning to speak out against it, Ali courageously refused to let himself be drafted to fight. This nearly cost him his career as a boxer, and no doubt took a lot out of it in any case, but it will go down in history as as act of transcendental bravery. It also showed his incisive intelligence, his extraordinary ability to get to the heart of the matter, when he explained, "I got nothing against no Viet Cong. No Vietnamese ever called me a nigger."
The other reason was more personal - and cultural. Muhammad Ali, in the early part of his career, was often criticized for his arrogance. But I always thought, and Ali proved it better than anyone, that modesty isn't all it's cracked up to be. Brashness, bragging can light up a room and the world - if the bragger has the goods to back it up, to fulfill the hyperbole and make it real. Ali backed up his braggadocio with punches and deeds and positions he took on the side of right way outside of the ring.
I've always counted myself lucky to have come of age in the 1960s - the age of JFK, MLK, the Beatles, Dylan, McLuhan, and Muhammad Ali. They and the impact of their lives will live forever.