Why is MOGO the soul of the green lantern corps?

If the theory holds true, Mogo is the soul of the Green Lantern Corp because of the nature of willpower. As it was said during Green Lantern: Rebirth, all living things are connected together through emotion, and each emotion is represented by a color. In the case of willpower, it is green, and is gathered through the central battery on Oa.

Now, it is safe to assume that Mogo is one of the oldest lanterns, and since it is a living planet, has a field which is made up of all living things on Mogo. That's a lot of living energy, and a lot of raw willpower. Logically, Mogo would be the soul of the Green Lantern Corps as being the second greatest localized source of willpower.

But that's not really the reason:

Mogo represents the ideal of the Corps, a massive living biosphere dedicated to protecting the life contained within, but also those who dwell in its sector. In essence, Mogo is the greatest example of the Corp's creed. Coupled with this is that every Green Lantern has at one time served with Mogo, which cannot be said even of Hal, John, or even Killowag.

The loss of Mogo would mean a shattering blow to the morale and ultimately, that's one of the reasons that the Sinestro Corp wants to destroy it.

All green lanterns train on and with Mogo. If the Sinestro Corps killed him it would have been a big blow to morale. He is considered the soul because every green lantern has a personal connection with him.