Why does water run out my compressed air line every time I turn the air on?

Water. This drives every compressed air user nuts!!! Every time they use an air tool, blow-off gun, or even fill the inflatable air mattress, water appears along with the compressed air.
The water is a problem if the compressed air is moving through a tool that can rust or be negatively affected by airborne particles, and, given enough 'fill-ups and empties' a significant amount of water will appear in anything which you inflate or run with compressed air.
Why?
Well, we know that relative humidity is the measure of water moisture in the air expressed as a percentage. At a relative humidity of 90% for example (really hot and sticky) the atmosphere is holding 90% of the total amount of water vapour it can hold. When the relative humidity in the air exceeds 100%, it usually rains.
That, unfortunately, seems to happen mostly on weekends! :-)
Now, let's take some of that atmosphere with it's 90% relative humidity and compress it. What do we want the final air pressure to be; 30 PSI, perhaps 45 Pounds per Square Inch? We will use 45 PSI as an example.
Free air, the atmosphere we breathe, has a normal PSI of 14.7 (or 15 PSI to make it easier math). So, we're going to take free air at 15 PSI and make it 45 PSI by compressing it. We will take three cubic feet of air at 15 PSI, and cram all three into the space of one to make one cubic foot of compressed air, now at 45 PSI.
If the relative in the atmosphere humidity is 90%, and we are cramming three cubic feet of atmosphere into one cubic foot, then the relative humidity of the compressed air will almost instantly exceed 100%. As a result, as long as the relative humidity of the air in the compressor tank is over 100%, it will rain in your compressor's receiver. That water will gather on the bottom of the receiver, and will ultimately fill it unless it's drained.
Then, every time compressed air is drawn from the compressor tank to your application, free water will follow it down your lines to your air tools, your workplace, your air mattress etc.
To make matters worse, compressing air generates heat. Air that would normally be saturated and not be able to hold any more water vapour at one temperature, can actually hold more than 100% humidity when it gets hotter. When the air inside the tank is hot, the compressed air that is flowing out of your compressor tank has a higher than normal humidity level.
What happens to air as it flows? It cools! What happens to the water vapour in the air as the air cools? It converts (condenses) back into free water.
More water!
The water generated by your air compressor, along with the water vapour carried along in the compressed air itself, both contribute to the water problem for the compressed air user.
And that's why you have water with the compressed air, streaming out of your air line every time you open the air line valve.