I thought no more of it until I was trying to catch my breath back at the end of a race. Why is there an addiction to racing bikes? The physical benefits, such as the release of endorphins, are well documented but perhaps the greatest enjoyment of racing is instead mental. The nature of bike racing is that it requires constant focus that banishes non essential thoughts. When your body is working at capacity and the terrain at it's most challenging, there is no room to worry about anything else. It demands your complete and undivided attention. As such it perfectly taps into the 'Power of Now'. This exhilaration we feel in these moments can be attributed to adrenaline or endorphins but perhaps it has as much to do with being completely in the moment mentally.
It could be argued that you don't need a race at all. All you need is a situation to help focus the mind on the immediate such as a demanding climb or slippery downhill. Racing, however, reduces the chances of calling time and easing off the gas, leading to the mental noise encroaching once more and reduced satisfaction. Racing leaves no room for anything else and for the commitment it demands, it returns a mental feeling that is enduringly addictive.