Why did Gandalf turn into the White Wizard

I've read the book, while it was a long time ago, I think Gandalf became Gandalf the White because he defeated an evil creature and his knowledge of magic expanded during his journeys with Frodo and co. His strength also increased and he became a lot more powerful... White Wizard is a rank held by powerful wizards, such as Saruman who was former leader or head of the wizard community, it seems Gandalf took his place after Saruman was corrupted by Sauron. Saruman tried to copy Sauron by building a great army and tower etc, he thought he could control Sauron and the ring of power... he started to fall and go crazy when he first began his research into the rings of power, he had good intentions to begin with, but as he learned more and more of the rings he wanted to control the world and own the Master Ring, essentially becoming just as evil as the Ring Wraiths. Shame he didn't become a Wraith, he'd be nearly as powerful as Witch King. There is really no way to Master the Ring, its Master is always Sauron, nothing can change that. People believe they can control the ring and the powerful ring wraiths but that's just their delusion. The most powerful are even more susceptible to getting dominated by Sauron... 90% of Saurons power is within the ring, essentially the Ring IS SAURON.

Whilst Gandalf was still Gandalf the Grey, Saruman ceased being Saurman the White and became Saruman the Many Coloured. After the battle with the Balrog, Gandalf was slain, and was sent back to continue on more powerful than before, as Gandalf the White.