Shem m'Shmuel (beginning of Tazria) writes that the machlokes R' Eliezer and R' Yehoshua whether the world was created in Nisan or Tishrei is l'shitasam of whether geulah requires teshuvah or not. Tishrei = the approach of winter, a time that can be used to prepare for the future seasons of planting and harvest. The world was created in a state where hachanos are required, where every end requires a means to get there, where geulah comes about only with a prior preparation of teshuvah. R' Yehoshua holds the world was created in Nisan; "ha'kol b'komasan nivre'u," things were created in an already prepared and formed state, the end is there even without the means beforehand. The geulah of Pesach in Nisan happened without preparation, without the need for Bn"Y to do much on their part.
The rasha asks, "Mah ha'avodah ha'zos lachem?" because the geulah from Mitzrayim was not one that was earned through our effort. It was chasdei Hashem, pure and simple. So why do we need to do anything now? Sit back and relax, it's a celebration of Hashem taking care of everything, no effort on our part required.
The answer is that a person has to be worthy of receiving chasdei Hashem. Imagine if someone offered to transfer a million dollars to your bank account but you had no bank account -- you would be out of luck. You may not need to earn the million dollars, but you need a kli kibul to receive it.