My favorite thing about Halo was how diverse it was and how it brought people together to have new experiences and openness of thought. Master Chief is you...he's me...he's all of us.
EDIT: for those of you who haven't caught on, I'm making fun of the current state of Halo
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Halo 1 and 2 were fun, but I also enjoyed ODST and Reach. ODST because you aren't a Spartan super soldier. Reach because it felt more real seeing all the dead Spartans in the later missions. You also get to see that Spartans aren't invincible, unlike the regular Halo series that have Master Chief wiping out an army of aliens like he is just some immortal killing machine.