Tao te Ching: 66

The great rivers and seas are kings of all mountain streams
Because they skillfully stay below them.
That is why they can be their kings.
Therefore, in order to be the superior of the people,
One must, in the use of words, place himself below them.
And in order to be ahead of the people,
one must, in one's own person, follow them.
Therefore the sage places himself above the people and they do not feel his weight.
He places himself in front of them and the people do not harm him.
Therefore the world rejoices in praising him without getting tired of it.
It is precisely because he does not compete that the world cannot compete with him.

Note the last line repeating Chapter 22. This is another example of Lao Tzu taking a geographical or physical phenomenon and lauding behavior that follows the same path. It's a carefully chosen physical analogy and not the root cause, of course. Lao Tzu did not study geology and decide that good leadership would be based upon it, but rather it is easy to imagine that the mountains contribute to the sea because their streams and tributaries eventually flow back into it, and it is the water from the sea that goes into the clouds and returns to the mountains as snow and frost that eventually break the mountain down.

We return again to hsüan-te, the profound and secret virtue, as the method by which the sage should lead. Put others first, and by their benefit you gain your own.