Question:

It seems that the coming of Moshiach is not under our control. Won't G-d send Moshiach whenever He decides to do so?

Answer:

G-d has given over the key to the redemption into the hand of every individual.

G-d indeed controls every aspect of the universe, particularly something as essential as the coming of Moshiach. However, He has given over the key to the redemption in the hand of absolutely every individual. R. Schneur Zalmen of Liadi, the first Rebbe of Chabad, teaches that "even if only one tzadik in a generation (and all Jews are considered righteous) would turn to G-d in perfect repentance, Moshiach would come immediately."

Maimonides likewise states as a clear halachic ruling that "If a person fulfilled one mitzvah, he thereby tips the scales in favor of himself and the whole world, and brings about redemption and salvation for himself and for the whole world."

G-d has given the key of redemption into our hands, because He desires us to be partners with Him in creation. G-d could have created a perfect universe and left it to run that way without any difficulty. However, G-d chose to create us, human beings with free will, so that we can play an active role in shaping the world, and making it into a G-dly and holy place.