Why does Gandalf...

[Olorin]

We must remember that Saruman was the head of the Istari and the White Council. If Saruman did not betray Gandalf, there probably would have been a meeting of the White Council where the fate of the ring could be discussed by the wisest and most influential people in Middle Earth. Think how much it would have helped if all the good guys of the land were coordinated to help the quest succeed. There would always have been a wizard looking after the fellowship, the powerful people of ME would have been able to better protect the Fellowship, etc. In addition, a better and safer route could have been plotted if the White Council met. It was only logical for Gandalf to seek help. Why would he take the burden upon himself when there were so many other powerful people that could help? Also, it was his responsibility to alert the influential people of ME about the Ring and the quest. It would have been stupid of his not to.