Your DSLR has a mind of its own. It's wayward and often has a completely different idea of how to change the settings to expose your scene correctly than what any sane photographer would use. If you've ever been in an automatic mode and found your night photo is a mushy mess because your camera decided to bump the ISO up to the Max, you will know what I mean. Yes, your DSLR cannot be trusted! But don't leave it for another piece of tech yet.
One of the reasons things go wrong in auto mode, is due to how a camera tries to interpret a scene. Take a photo of an all white object that fills the whole picture frame, then take a photo of an all black object right after, both in auto mode. (I did this with a serviette and my friend's mobile phone today) Guess what? Your camera will have tried to make both objects look almost the exact shade of boring grey. But why? A photographer wants white things to be white but when the camera is confronted with a pure white scene, it will try to make it into a dull grey. That's because cameras are programmed to give you an average exposure. I will probably go over this in more detail in future as I'm currently in a dark room with a baby alseep on me, but for starters, when you are doing a night scene, don't use all automatic. Set a low ISO, use a tripod and dial in 1 or more stops of negative exposure compensation if you use shutter or aperture priority mode as a night scene should look like it is shot at night. Without the exposure compensation, your camera wants to make the day black sky into boring grey! For the extra cool factor if you are in full manual mode, dial in f16 or greater to get lights with that classic starry effect!