I'm still seeing explanations being pulled out of the air, so to speak. I'm a 100-hour student pilot, and let me tell you what every pilot must absolutely learn about stability in the pitch axis, because it's life-and-death knowledge. You don't need a PhD in physics to get the point.
Let's start with a simple ordinary small airplane like a Cessna 172, like I learned on. You must be aware of the center of gravity (CG), and it must be forward of the center of lift of the main load-bearing wing. It has a range, with a forward limit and an aft limit. You must be able to do a simple calculation based on the weight of fuel, the weight of passengers, and luggage, and their position forward or aft. You're basically adding up torques due to weight and moment-arm, and this tells you if your CG is within limits.
Why is the CG forward of the center of lift? Because that means the tail has to push down. (It is actually an inverted wing.) This creates a pitch-wise torque between the main wing and the tail, and the strength of that torque is proportional to speed squared.
Why do you need that? For stability of speed and pitch. If for some reason the plane hits an updraft and pitches up, what happens? Since it is going up, it loses speed, which lessens the pitch-wise torque, which allows the heavy nose to pull down, so it returns to its original pitch and speed. Same if it hits a downdraft - it speeds up, more nose-up torque, and it returns to original pitch and speed.
What happens if you move the CG too far aft, by putting lead bricks in the baggage? You no longer have that stability in pitch. You have to concentrate really hard to keep it level. The slightest finger pressure on the yoke, and you are zooming up and stalling, or zooming down and overspeeding, and oscillating chaotically between the two. If you're not careful, you can get nose up and slow and actually start a backslide, that you can't get out of, all the way to the ground.
Of course, if the CG it too far forward, you get different problems, that aren't too hard to figure out.
Fun?
Same goes for any simple "aircraft" like your paper board. There needs to be a speed-dependent torque causing it to pitch up, and a forward weight pulling the nose down. Even if the board is flat, its center of lift is forward of the center, so there is a torque. Just because a "flying wing" doesn't seem to have a tail, its trailing edge has enough of an upward tilt to provide that torque. Some planes have a canard instead of a tail, but it's the same idea - faster speed = greater nose-up torque, and the CG is always forward of the center of lift.
Exception: Military jet fighters. They prefer instability so they can maneuver very quickly. They have computers manage the controls, because a human could never do it. They look down on plane-old planes, calling them "God-fearing" aircraft :)