It was as if Stanislaw_Lem wanted to include some aspects of the Gaia_hypothesis into his fascinating story. https://en.wikipedia.org/wiki/Gaia_hypothesis#Overview
Solaris isn't just a water-world, its a world of consciousness with an aquatic psychic brain. https://en.wikipedia.org/wiki/Gaia_hypothesis#Regulation_of_oceanic_salinity
The aquatic brain of Solaris is a very powerful form of sentient life. That means it can play serious mind-games with visiting humans. The nature of Solaris might not be benevolent or malevolent. However, its aquatic mind is very protective against any visiting life forms.
https://en.wikipedia.org/wiki/Solaris_(novel)#Plot_summary
Of course any good story can have various interpretations.
https://www.newyorker.com/culture/culture-desk/the-beautiful-mind-bending-of-stanislaw-lem
https://www.goodreads.com/book/show/95558.Solaris
http://www.conceptualfiction.com/solaris.html
https://www.tor.com/2019/08/01/embracing-the-impossible-puzzle-of-stanislaw-lems-solaris
A few visual depictions of the story have been made.
https://en.wikipedia.org/wiki/Solaris_(1968_film)
Most regard the 1972 movie as the best version. However, even it didn't fully meet the approval of Stanislaw Lem.
https://en.wikipedia.org/wiki/Solaris_(1972_film)#Plot , https://www.youtube.com/watch?v=wT8Y7DGQn7M
Solaris film analysis https://www.youtube.com/watch?v=xKxC5iZjrmI
