Why drag accelerates and lift eventually stalls
As angle of attack (AoA) increases, lift rises nearly linearly up to about 15 degrees. Beyond that, the separation of airflow over the wing accelerates, causing lift to plateau and then drop sharply at roughly 18-20 degrees, a condition called aerodynamic stall. Meanwhile, drag rises smoothly at low angles then accelerates at higher angles as the separated flow creates bluff-body drag.
The key difference in rates of change means that at low angles of attack, the aircraft gets lots of lift for little penalty in drag (efficient flight), while at high angles, the drag penalty becomes enormous. A typical airliner cruises at 4-6 degrees angle of attack where lift-to-drag ratio is near maximum. Attempting to fly much slower requires increasing angle of attack, which quickly reduces efficiency and approaches the stall boundary.
Optimum angle of attack and design trade-offs
The best lift-to-drag ratio typically occurs around 10-12 degrees angle of attack, where most aircraft cruise. At lower angles, the wing is too inefficient relative to available lift. At higher angles, drag penalty outweighs additional lift. This optimal angle is not arbitrary; wing design is optimized around this point through camber (wing curvature) that generates lift at near-zero angle when the aircraft is level.
Aircraft must stay within a safe margin below the stall angle: a commercial plane at cruise operates at roughly half the stall angle, leaving substantial buffer for maneuvers and wind gusts. High-performance aircraft might cruise closer to the optimal ratio for efficiency, but this leaves less safety margin. This is why aerodynamic stall is so dangerous: exceeding the stall angle suddenly reduces lift drastically, and the separated flow that caused the stall persists even if angle of attack is reduced, requiring nose-down pitch to regain control.