Oh, I must have forgotten writing that. It's shortsighted, not selfish. Belgium will get destroyed either way, except in here it will lose its sovereignty to boot as it has demonstrated its uselessness as a buffer zone. In fact, as a belligerent in the CP side, the Entente forces can destroy every Belgian city, port, infrastructure etc. without consideratation for collateral damage.
All this achieves is that Belgium becomes a legitimate military target in and of itself to the Entente instead of a speedbump and that trench warfare happens a few miles further southwest.
Even if the CP win through ASB logistical handwaving and Entente idiocy, why would Germany leave Belgium alone a few years down the line when it can make it another fine addition to the German Empire? It can be argued that Belgian sovereignty does not need to be respected since it broke its neutrality and cannot be trusted to keep its treaty obligations. Who is going to stop the Germans? The French?
OTL Belgium made the less bad of two very bad options.
If they went to the Entente and won, that was the best scenario as they kept their obligations and proved their usefulness as a buffer zone. If they lost, they might still be kept by Germany to funnel French counterattacks in the future down a narrow line of attack.
If they go to the CP, they throw away their only value and become an unreliable and untrustworthy actor that might as well be annexed by Germany, or they lose and get both their land destroyed and their sovereignty revoked as they have no value whatsoever as a buffer zone.