Why do soldiers have to shave?

I rang my best mate last night and the conversation turned to this thread. when I asked him why he saved during his time in the RAF he gave me the exact reason - we were told to and if we didn't we got shouted at. There may be operational reasons, but when you get down to it, the reason soldiers, sailors and airmen shave is because they were told to!

unsurprisingly, my mate now has a beard and the 2 other ex-servicemen I work with also have beards. Personally, I cannot stand them even though my dad, my grandfather and my uncles all have or had beards!
 
Back in the day when I was in the army we were told it was due to respirators fitting correctly. Never served with a Sikh so not sure how that goes. Interestingly I did a multinational tour whereby the Dutch, Belgian and German soldiers could grow beards. Also I served in a hot distant country on operations where there was just not the water to shave so we were told not to bother from up high.
 
Sorry of topic, but cool, I hadn't realised there was another pestie on board!