My mind unknowingly paints bright light green on those leaves. What makes curves artistic in our mind? Still thinking..

Ganesh, curves rather than straight lines convey a very different feel to us: gentle, smooth curves are soothing (e.g. human body contours, grass blades) and jagged, random curves are alarming (e.g. crashing waves, fire). Their artistic nature comes from their ability to invoke such feelings in us, IMO.