Why? He is the solid grounding for Frodo. He is the character that must discover how to support Frodo, emotionally, mentally and psychologically. This is certainly not typical of the behaviour of Hobbits.
And also in the events at the end of the Return of the King, (the book, not, the movie), Sam shows incredible leadership and determination. I won't mention what as it is sort of a spoiler for those who have only watched the movies and are still to watch the third installment.
He is the only character in the Lord of the Rings to have posessed the ring and not wear it.