You're a good motorcycle owner, keep your battery on the proper maintenance charger, take care of your bike like a true anal-retentive, but here you are. Dead in the garage. A weekend ride quashed or at least delayed. How come? There are many reasons for battery death. Let's look at a few.
1. You have a leak circuit somewhere. This is simply a low-level current drain on the battery that slowly sucks it dry. If you've had the bike on a charger, this drain would have to be bigger than the chargers capacity. Leaks can come from accessories, alarms, GPS power supplies, or other sources. The way to check is to remove the negative battery cable and place your volt-ohm meter in the current (amps) mode. With the key off, place the meters leads between the negative cable and the battery's negative terminal. The current draw should be zero, but a small drain on the order of a few milliamps is acceptable. A drain of an amp or more is reason for further investigation.
2. Poor ground connections between the battery and the frame can inhibit charging. Carefully check all high-current connections by removing them and cleaning them lightly with sandpaper or Scotchbrite.
3. Your voltage regulator/rectifier is dying. Happens all the time to high-mileage bikes. First check all the connections from the alternator to the regulator--some bikes are notorious for frying these connectors--and then consult the service manual to check the resistance on the alternator and regulator wires. But understand that the most commonly used regulators rarely outlast the bike. (A few motorcycles have more sophisticated regulators that tend to last longer.)
4. Too many accessories. Check the voltage at the battery with the engine running at some modest rpm (say, 3000) and all the accessories on. If you don't see 13.8 volts or better, the charging system isn't keeping up with your add-ons. Some bikes have very little extra juice to spare, so you might have to shed a few of your accessories or seek a more powerful alternator.
5. Well, it's just done. Batteries don't last forever, and some installations are harder on them than others. Heat and vibration can kill even a carefully maintained battery. Even if a battery seems to hold a charge, internal damage from broken plates or poor internal connections can be fatal. How long should a battery last? If you got four or five years, count yourself lucky.