Think of the restroom as a sanctuary. For many folks, it's one of the few places where they can get a few moments' break from the hustle and bustle of life. For others, it's simply a place to take care of some highly necessary and completely personal business. Both groups would like you to kindly check your conversation at the door.
This is particularly true at the urinal. Just because you're standing next to someone and staring at what's probably a blank wall in front of you, it isn't an invitation to start yakking about whatever inane thoughts are on your mind. Nor is it a place to conduct business, no matter how pressing the topic might be. For one thing, you never know who else might be in there lurking behind a closed stall door. If you're the one in the stall, the only time you should strike up a chat with your neighbor is if you need to ask him to pass over a roll of toilet paper. Otherwise, save the office gossip for the water cooler, the break room and the happy hour bar.