If you look up the etymology of *samsara*, it's composed of "*sam*" (together) + "*sara*" (course, motion, stretching out, extension). So, it's like moving or flowing together like a river.

If *samsara* is a river, then craving (*tanha*) is like the river current. It's the force that moves the river and it's the cause of suffering (first noble truth).

> This was said by the Blessed One, said by the Arahant, so I have
> heard: "Suppose a man was being carried along by the flow of a river,
> lovely & alluring. And then another man with good eyesight, standing
> on the bank, on seeing him would say: 'My good man, even though you
> are being carried along by the flow of a river, lovely & alluring,
> further down from here is a pool with waves & whirlpools, with
> monsters & demons. On reaching that pool you will suffer death or
> death-like pain.' Then the first man, on hearing the words of the
> second man, would make an effort with his hands & feet to go against
> the flow.
> 
> "I have given you this simile to illustrate a meaning. The meaning is
> this: the flow of the river stands for craving. Lovely & alluring
> stands for the six internal sense-media. The pool further down stands
> for the five lower fetters. The waves stand for anger & distress. The
> whirlpools stand for the five strings of sensuality. The monsters &
> demons stand for the opposite sex. Against the flow stands for
> renunciation. Making an effort with hands & feet stands for the
> arousing of persistence. The man with good eyesight standing on the
> bank stands for the Tathagata, worthy & rightly self-awakened."
>
> [Iti 109][1]



  [1]: https://www.accesstoinsight.org/tipitaka/kn/iti/iti.4.100-112.than.html#iti-109