At a frozen-in-time (1/2 cycle) moment, L1 is positive and N is negative, and sat the same time, N is positive and L2 is negative. Overall, L1 is positive and L2 is negative. Just like two batteries in series.
1/2 cycle later, everything is reversed, which is okay, because it's all reversing in sync, or in phase. Just like two batteries in series that are both being reversed in sync, or in phase. The voltages are additive.
If and only if you choose YOUR reference of an endpoint. And it is only "just like two batteries" if you want to use the markings and terminal caps someone else chose and if you think a static system can equate to a dynamic system.
AC is not just like a battery stack. Imagine a battery with generic caps on each end and no +/- markings with the inside continuously cycling. You can use it as a positive supply or negative supply at an instant but no polarity at one instant is ASSUMED to be a "correct" direction indicator like you keep thinking.
When you keep thinking of a battery, you keep ASSUMING there is a certain polarity and positive direction implied and there is not. Someone has marked + and - on the batteries but you could not mark the terminals if the inside was constantly changing.
The AC wave can begin life as a positive or a negative signal. There is NOTHING that says it has to be one or the other. It is the user's choice to pick a reference direction.
If you keep standing on the equator and facing east or west, you will think left or right must always be directed north or south. If that is all you know, you will never be able to understand that you can stand on the North Pole and both left and right can be directed south.
It is a fundamental concept that most engineers understand about physics, or should have understood at one time. It is one of the very first things taught in basic electrical physics. A voltage reference is a choice and picking one over the other is not against any physical law but is actually how the physical world works.
But if it makes it any easier for you to think about, you can use a battery as a positive supply or a negative supply. I can't begin to tell you how many times I've used a "positive" voltage regulator as a negative supply in a circuit.
A rise in one direction is a fall in the opposite direction. You get to choose the reference direction and either way is valid.
It really is that simple.