Here are two comments on r/askhistorians saying water was one of the reasons [1], along with natural resources in general [2].

Of course that's not the only reason; other answers make clear there were a lot of factors. Now though, Tibetan water is a huge issue, including for other countries downstream of Tibet including India and Pakistan [3]. There are no water treaties between these countries; China has been building a lot of dams and is planning a massive 600-mile pipeline to divert Tibetan water that would otherwise go to India [4].

Tibet is the third largest repository of fresh water after the two poles [5].

[1] https://www.reddit.com/r/AskHistorians/comments/35fuz5/leavi...

[2] https://www.reddit.com/r/AskHistorians/comments/2giaje/was_t...

[3] https://www.equaltimes.org/china-s-water-grab-in-tibet-risks

[4] https://qz.com/1114843/chinas-grand-plan-for-the-brahmaputra...

[5] https://www.theglobalist.com/tibet-and-21st-century-water-wa...