Here's why:
* Common nouns refer to general, non-specific things. They are not capitalized unless they are at the beginning of a sentence.
* Proper nouns refer to specific people, places, or things. They are capitalized.
Examples:
* Common nouns: season, dog, city, book
* Proper nouns: Spring, Fido, London, The Lord of the Rings
Since "season" refers to a general time of year, it is a common noun.