It says vehayisa ach semeach, so let us not squander the simcha we acquired through the holidays and keep it, at least in a smaller measure, for the rest of the year.
GH, It says, the same as when Av enters we duminish in simcha, so when Adar enters we increase in simcha. What is the comparison? Says Rav Yosef Baumgarten in his sefer Pardes Yosef, that we certainly fast lashem shomayim but eating lashem shomayim needs extta effort. See the topic Yom Kippur like Purim.