Here's my thought: you draw with shapes. Think about that. You don't draw with expression lines or sketch lines or lines of any sort. What you convey are shapes. That's why you and I will never draw exactly alike... our shapes will vary, not matter how hard we try to remain the same.
That's not a bad thing; it's a good thing, really. How we convey our shapes reveals our style. Our style. Everyone draws shapes a little differently.