Mem represents water, expansion, and the male, while Shin represents fire, form, and the female. Above we have the fire of the sun and the stars. Below we the earth whose covering is mostly water. In between, we have the air. On the one hand, it seems natural to associate the heavens above with spiritual fire and the earth below with water, but on the other hand, one could argue that since "mother earth" has greater form than either the atmosphere or the sky, it should be represented by the female letter Shin. In trying to explain how the earth takes on a masculine role, we can derive a valuable lesson. First, notice that all things have both a male and female side. We see this in the way in which each Aleph-Mem-Shin triad is further subdivided into another triad. In this particular case, the association of a masculine letter with the earth teaches us that we need to take the initiative before heaven will respond to help us. This is explained in the Zohar as follows:
"A MIST WENT UP FROM THE GROUND, to repair the deficiency below, by "watering the whole face of the ground". The rising of the mist signifies the yearning of the female for the male. According to another explanation, we supply the word "not" from the previous clause after "mist", the meaning being that God did not send rain because a mist had not gone up, etc., it being necessary for the impulse from below to set in motion the power above. So vapour first ascends from the earth to form the cloud. Similarly, the smoke of the sacrifice rises and creates harmony above, so that all unite, and in this way there is completion in the supernal realm. The impulse commences from below, and from this all is perfected." -Zohar I, 35a
We also derive this same lesson from the following verse from Psalm 85:
"Truth shall spring from the earth; and righteousness shall look down from heaven. -Psalm 85:12"
There is an inteplay back and forth between giving and receiving. No one remains in either mode forever. Heaven gives to us from above, but we must initiate the request before the universe responds.