Here's two reasons:
1. Helmets give the cyclist a false sense of security.
2. Helmets give drivers a false sense of security.
You may think [1] does not apply to you, and possibly it doesn't but people are incredibly bad at judging that kind of thing. It's very likely that you take more risks when wearing a helment.
The second point is far more important and it's not something you as a cyclist can do anything about. Studies have shown that cars pass closer and faster to bikes when the cyclist is wearing a helmet. On some subconscious level they see the cyclist as being less vulnerable and hence they drive more dangerously around them.
For these reasons I discourage my three daughters from riding helmets when they cycle and I don't wear them myself.
However, even if one discounted both these reasons, mandatory helmets are horrible on principle. Its my own life I may be putting in danger, so if you want to wear a helmet, go ahead, if you want to tell other people to wear a helmet, go fuck yourself.