The Mishna opens Mes. Megillah with the phrase "Megillah nikreis..." and lists the possible days to read the megillah. Why does the Mishna use the passive voice, "nikreis...", instead of the more direct "korin es hamegillah"? The Turei Even answers that the Mishna could not write "korin..." because each location, kfar, krach, and ir, reads on a different day - you cannot instruct to read, "korin..." because that has to be qualified by the location of the reader.
The Ritv"a explains that the Mishna uses the term "nikreis" because the Megillah is not read by everyone - we listen to the baa'l koreh and are yotzei through shomea k'oneh. However, continues the Ritva, the Mishna in Brachos uses the term "korin es shma" because each person must read shma independently and not rely on shomea k'oneh. This idea is usually quoted in the name of the GR"A in Shnos Eliyahu on the first Mishna in Brachos. Of course, the question to think about is why keriyas shma cannot be fulfilled through shomea k'oneh???
R' Tzadok haKohen writes that the essential nature of the day of Purim is our strength to defeat Amaleik, which is the power of the yetzer hara. "Ilamalei ozro lo yachol lo", without Hashem's help we would not have the power to accomplish this ourselves. Therefore, "megillah nikreis..." it is not our active efforts which bring about the megillah, but our being recipients of siyata d'shemaya. "V'zichram lo yasuf", the same occurs each year, even in our times.