system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why did Saruman the White defect to Sauron?
gpt
Initially Saruman wanted the One Ring for himself. In somewhat simplified terms, he was jealous of the respect and admiration given to Gandalf. So he wanted the Ring and its powers to make himself greater.

In the events leading up to and including The Hobbit, he hasn't yet begun to be controlled by Sauron, but he does want to leave Sauron alone in the hope that the lost One Ring would turn up as well (hence, he counsels against attacking Dol Goldur at the 2851 White Council).  Saruman then goes off to start looking for the ring, at Gladden Fields where the Ring was lost when Isildur was killed.

Then at a later White Council meeting, at a time when Bilbo has found the ring and events are moving forward swiftly, Saruman switches course and says they should attack Dol Goldur to keep Sauron from reaching the river. Sauron leaves DG, the Battle of the Five Armies happens.

Only 50 years after those events does Saruman use the Palantir, establishing the mental connection to Sauron.

So you could say he doesn't really "defect" since from very early on, he's trying to gain power for himself.  He was never a truly stand up guy, in other words.  But, he only begins to be controlled directly because of his desire for power when he uses the Palantir.

Edit: pushes glasses up nose, adjusts pocket protector. Thanks for the gold!