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Conductors
The reason it's worth paying attention to the location of your antenna in relation to your aluminum siding is the exact same reason many antennas are made out of aluminum themselves -- aluminum is an excellent conductor of electromagnetic radiation, specifically radio waves. In fact, according to the 2011 edition of "The ARRL Handbook for Radio Communications," aluminum is the fourth most conductive metal, after silver, gold, and copper. As a result, since most antennas designed for ham radio are intended to be installed "in the clear" -- that is, free from obstructions and buildings -- the aluminum siding can have adverse effects on your reception and transmitting performance.
Outdoor Concerns
Putting your antenna too close to your aluminum siding, or any other large metallic object for that matter, can cause a variety of issues resulting in degraded reception. The large, flat surfaces of the siding are perfect for reflecting the incoming radio waves, which can result in the phenomenon of multipath distortion where the radio signals reach the antenna by more than one path. This results in signals having echoes and other unpleasant audio artifacts. The presence of a large amount of nearby metal can also detune the antenna, which results in reduced efficiency and weaker received signals.
Indoor Concerns
If you're in a location where your ability to put up an antenna outdoors is restricted by a homeowner's association, you may have considered putting your antenna indoors and out of sight. Unfortunately, if your house has aluminum siding, you will find this to be an exercise in frustration. At most radio frequencies, a building wrapped in metal will act as a Faraday cage, causing reception of even strong signals to degrade to the point of being almost inaudible. To see an example for yourself, try this simple experiment: listen to a small portable radio while placing a metal kitchen strainer or window screen mesh over it, and note the change in signal strength.
Solutions
All is not lost, however, and there are some simple and effective remedies that don't involve ripping the siding off of your house and starting over. In general, raising an antenna higher in the air is almost never detrimental to performance, so just raising it up and away from the side of the house can make a substantial difference. Barring that, antenna manufacturer Alpha Delta Communications recommends that their antennas be installed at least 15 feet from any metal house siding. If going higher isn't the answer, even a lateral move can still be an improvement.