But life isn't an unnecessary risk. And it's your life, you control your own risks.
When you're controlling the risks of a creature who has no say in the matter, then you need to start weighing out the risks to rewards.
Lets do this for example
Cohabitation is a risk. You agree with that don't you? As you have countered with saying life is a risk as a justification.
So since we agree its a risk, lets dig a bit deeper.
You buy a dragon. A month goes by and things are going great. You decide to buy another dragon and contemplate housing it with your current dragon, which, is a risk.
Now, will this dragon that was doing so well before, start doing better being housed with another dragon? No, it wont. Will it start acting worse? Well, I'm not going to speculate. It could or it could act just perfectly fine with his/her new tank mate. The only real reward is for the human, not the dragon.
So, for the dragon, there is a risk for no reward. This doesn't mean that the dragon will get mauled to death by its new tankmate, it just means the dragon gets no benefit from this very serious risk. Much like Russian roulette, but without the adrenaline rush.
That, is why I am against it. Not necessarily because its a risk (like you said, life is full of risks), but because its a risk with no reward to the well-being of the dragon.
If you have success housing dragons together (and I say this to all who successfully house together) then i'm happy for you and your dragons. The chamber you fired wasn't loaded. But I do not condone one person who lived after playing Russian roulette telling another to "go ahead and do it, I did it and I was fine!"
-Brandon