If I can use an oft-accessed plot, Sam in The Lord of the Rings seems like a supporting character through most of the novel, but that exposition builds his character--the humble, dutiful friend/servant. It isn't until the final third of the trilogy, when Sam is not only the physical support for Frodo, but the mental and emotional support, that we see his self-sacrifice and love for not only his friend, but through him the world at large. He willingly sacrifices his future (his life really) in order to help his 'Master' finish his goal...he never sees himself as a hero. His isn't the false humility, but the actual kind...there isn't a bit of selfishness in him. Anyone who has loved someone and cared for them through illness and infirmity, suffered silently in order to make things easier on someone else, or given up their own precious time in order to give someone else a break or respite from suffering, can relate to the character...empathize with him, really.
It is no surprise that most main characters in fantasy and science fiction are 'everyman/everywoman' characters. Can you really relate to Gandalf, or Sam? Dumbledore, or Harry Potter? Han Solo, or Luke? It is the everyday person we relate to the most, because we are everyday people. None of us are Aragorn, or Morpheus, or the Tin Man. We're Dorothy. We identify with the weakest individual in fiction because regular people are the weakest in most fiction genres. It makes us enjoy it that much more when we see the average Joe character struggle, strive, fail...then succeed.