Tolkien isn't very good at explaining his villain's motivations; they tend to just 'turn evil' for the sake of the narrative. Sauron, and his old boss, Morgoth, in particular, are hard to work out.
For those who don't know the back-story, here it is: Eru (basically God) makes the universe with the help of his Valar and Maiar (basically angels), but one Valar, Morgoth, mucks around and starts making the world in a weird messed-up way. Why does he do this? No one knows, but the canon explanation is narcissism. Morgoth continues to muck around, and eventually, everyone just gets fed up and calls him 'evil,' and a whole bunch of wars happen until they capture Morgoth, chain him up, and shove him into the void.
Not everyone is against Morgoth though, the Balrogs are actually fallen Maiar, as is Sauron.
Now, there are two important things I have to mention before I go on; it is foretold that Morgoth's return will bring about the final battle, the end of the world. As well as this, there is 'Morgoth's Ring.'
Quote: 'Just as Sauron concentrated his power in the One Ring, Morgoth dispersed his power into the very matter of Arda, thus 'the whole of Middle-earth was Morgoth's Ring'
This is what interests me about Sauron. Why did he want to take over the world? We can go with the canon reason, which is again narcissism, but I think Sauron's ultimate plan was to bring back his old master and bring about the end of the world. What he intended beyond that is just more speculation.