As a rule of thumb, if your proceeding forward will cut off another driver, you don't. If one guy wants to go straight and another wants to turn and they both got there at the same time, the dude who wants to go straight should go first.
It's the same reason you might be able to turn right onto a two-way after stopping at a red light but cannot turn left: you'd be cutting off the left-going-right traffic. I may not be explaining it as well as my driver's ed teacher did, but I get it so that's all that matters to me