He was sick of multinationals like Coke buying water for a couple of cents a mega-liter. Coke would then add some sugar and color to it before selling it back to the people for dollars per liter. The expensive purification of that water was paid for by tax revenue.
Chávez decided this was not right, and put an extremely heavy tax on multi-nationals buying "resources" from Venezuela and doing that.
Personally, I think the world would be a better place if that was the law everywhere.
On a related note, Juneau, the capital of Alkasa needs a new water purification plant, theirs is running beyond capacity. The proposal was to increase tax so everyone pays for it. It came to light Holland-America (cruise line) buys millions of gallons for a tiny, tiny fraction of the price that locals pay. When it was suggested to Holland-America they should pay a little more to help fund the new purification plant, Holland-America simply threatened to stop bringing cruise ships to Juneau. So now the residents pay more tax so that Holland-America can continue to buy millions of gallons for much cheaper than the locals pay!