Pressure-volume curve: Elastic Recoil Pressure don't make sense

[brownie_souffle]

From pressure-volume curve of the lung and chest wall (attached photo), I don't understand why would the elastic recoil pressure of the lung is initially negative then becomes positive above 30% of vital capacity when the lung volume increases from residual volume?

 

What I initially thought was as the lung volume increases, the elasticity of the lung would increases (thus increasing elastic recoil pressure) because the lung is being stretched more. However I don't understand as to why should the elastic recoil pressure starts at such a negative value in the beginning?

 

Can someone please help me clear up this confusion?