It is just my personal theory...

But I think letting female dogs go into heat without breeding them makes them more aggressive.

I really don't have a large enough sample to make this judgment. I really should go a do some research, but I am facing down a weekend that includes two females coming out of heat, one in the middle and one that is pregnant. I believe this will be the first case of estrogen poisoning here at the ranch.

I have seen nice dogs become psycho during their heat cycles. They make my PMS look like a walk in the park.

They are mean to the male dogs - "Sorry boys, you are just not up to my standards!"

They are mean to the female dogs - "If I am going to breed, that means I am the alpha female, and I just want to make sure that everyone knows it!"

It seems to me that the aggression continues after the cycle ends, particularly to other females. The level of aggression is also notched up every cycle. Now I could be wrong, I have mean females that have been neutered. You may have had your dog go into heat and see no personality change. I think I probably see the worse behavior because I get to see the dogs in a social setting. And face it, no female of any species looks its best when trolling for a mate.