Sidequests need to be one of two things. Either they need to be so different from the main quest as to feel unique, or they must serve to expanded on and enhance the fiction, scenario, world or main quest; essentially acting as pieces of a larger picture to flesh out the narrative.
Good examples of the former are found in The Witcher 3. Sidequests in TW3 rarely have anything to do with the main quest, but each features as a sort of a day-in-the-life of the witcher Geralt and each is its own self contained narrative with a depth and complexity all its own. In each one you'll meet new characters and new plot lines in entirely unique situations. This not only makes them feel unique from each other, but different enough from the main quest as to make pursuing them worthwhile.
A good example of the latter type is found in Fallout: New Vegas. New Vegas establishes this rich scenario and world that is introduced in bits and pieces of exposition as you go about your adventures in the Mohave wasteland. Every character and settlement has their own place in this rich world and all of the events and conflicts that are taking place. All of this is presented in sidequests, with each one presenting a conflict that conveys further understanding of the situations taking place across the Mohave. Whether it's helping a town fend off an assault from a violent gang that escaped from the overrun New California Republic prison down the road, or helping hapless New California Republic soldiers secure their supply lines, personnel, communications network and moral against the murderous Caesar's Legion, to helping independent New Vegas residents protect themselves from overbearing New California Republic officials or helping the benevolent Followers of the Apocalypse assist the downtrodden residents of Freeside... every situation you encounter paints another piece of the picture of the world of the Mohave and its various inhabitants. By actively participating - of their own accord and by their own initiative - in the events taking place, and by making final decisions that affect each situation and the characters involved, players come to intimately understand and develop a vested interest the world of New Vegas and its people.