Is your relationship with your next-door neighbour running a bit thin, because you are constantly asking him if he can "jump" your car's dead battery? If you can't work out why the battery should be behaving this way, then you need to take time out of your busy day to figure it out once and for all. If you don't, you're not going to get an invite to that neighbourly barbecue at the end of the month! Seriously, though, what can be wrong with your battery, especially as it is quite new?
First Things First
Start by running some simple checks and having a good look underneath the bonnet. Has a certain amount of corrosion built up around the posts and terminals, or on the end of those all-important straps and leads? If not, you should still try taking the connections off and cleaning the terminals anyway, just in case something wasn't seated properly.
Triggering Light Switch
If all looks good there, perhaps the problem is a little more obscure. If your vehicle is somewhat older, the issue could be related to its interior light, believe it or not. A typical car is always on standby, with the electrical system ready to spring into action whenever needed. These systems and features are designed to go into hibernation mode when the car is stationary, so that the battery is not drawn down needlessly. Many of these circuits and components take their cue from the interior light. The light will switch on whenever the door is opened and will go off when it is closed. This deactivation controls a lot of the other peripheral circuits and tells them to turn off, as well.
It is possible that this switch is faulty and is not sending the other components into hibernation as it should. Sometimes, the switches can go bad, and when this happens your battery could easily drain down to zero overnight.
Check the operation of the light by opening and closing your door. Does everything appear to be working okay? If the bulb appears to have failed, this would cause the same result.
Faulty Internal Relays
If everything seems to be okay there, perhaps an electrical relay is stuck "on" in the system somewhere. If this were to happen, the current would flow constantly when it's not needed and would drain the battery. This can be somewhat more difficult to find, however, and will invariably require a visit to a mechanic in order to do a series of specialty checks and car servicing.