well, in an unopened bottle or can of soft drink, there is an equilibrium of CO2 in aqueous and gaseous form, some goes into aqueous and some comes out;
like so:
CO2(aq)<-->CO2(g)
when you open the bottle or can, the gaseous CO2 is released, shifting the equilibrium to the left(?), so more gaseous CO2 needs to be made to restore equilibrium. Le Chatelier's principle states: if a chemical system in equilibrium changes in temperature, volume or pressure, the system will shift to minimise the disturbance. or in lay terms: if a system in equilibrium is disturbed, it will adjust itself to minimise the changes.
seeing as opening the system (soft drink bottle) lowers the pressure (they are packed at approx 2 atmospheres), CO2 comes out of the soft drink (aqueous to gaseous forms) to try and re-establish equilibrium (minimising disturbances). this makes the soft drink go flat.
Does that help any?