Saruman was just trying to find a friend, albeit his friend corrupted him in the long run- but look at the "White Wizard's" peers:
The 2 blue wizards who were off - in the east- doing who knows what. Radagast, the hippie stoner who never showered and wouldn't stop eating mushrooms. Gandalf, the homeless stoner who kept hanging out with all those carnival folk who never wear shoes.
Saruman gets sick of it, and gets on an AOL chat room and meets the Dark Lord. Someone who loves Metal, puts flames on everything and has a bunch of badass minions with piercings and muscles and giant black towers just like Saruman's. So he makes friends.
Next thing Saruman knows, his old stoner roommate rolls up and says, "Hey, I'm going to kick this Sauron guy's ass bro, you gonna roll with me?"
So, after some debate- lets Frodo pick the way to go. And Gandalf is terrified to go the way he picks, though he doesn't remember why. This of course gets them into probably the worst case scenario- as he forgets how to even get in, forgets which way they should go, snaps at others who question him- and doesn't remember that the Balrog is there until it's basically right on top of them.