Tao te Ching: 76

When a man is born, he is tender and weak.
At death, he is stiff and hard.
All things, the grass as well as trees, are tender and supple while alive.
When dead, they are withered and dried.
Therefore the stiff and the hard are companions of death.
The tender and the weak are companions of life.
Therefore if the army is strong, it will not win.
If a tree is stiff, it will break.
The strong and the great are inferior, while the tender and the weak are superior.

Consider Chapters 22 and 64. Lao Tzu says

To yield is to be preserved whole.To be bent is to become straight.

Which means that flexibility is essential. Here he explains his reasoning. It is interesting to me that the logic appears to hold up only superficially. We do not expect, in this day, that the stiff and hard (as companions of death) mean that anything stiff and hard are likewise associated with it. And yet, consider the skin. Think of how tough it must be to keep your limbs inside. Protect you from germs and wounds and temperature changes. Think of how hard it is to actually tear your flesh accidentally. It doesn't come off when opening a door knob or walking along the floor. Yet a small, hard instrument like a needle or a knife can rend it. That's death trying to overcome life. And it's possible for death to spread those qualities of stiff and hard. Your skin will become drawn, like parchment. it will split and crack and lose its ability to protect.

Our logic and reason says that those qualities are associated with plenty of other things. Yet, think on it; we have a car made of metal and say it protects us. But the protection of the car relies upon its ability to have shocks that bounce and a framework that crumples in some places to protect the passengers. It too is comparatively tender and weak.