AgileGSD: If head collars made it physically impossible to pull, which seems to be how you believe it works than no dog would be able to pull against them.
I didn't say it makes it impossible, I said it gives you the ability to control the head, and if you can properly control the head, you can properly control the pulling. You don't just slap it on and BINGO, the dog stops pulling. It takes effort and concentration on behalf of the human. If the human doesn't use it right, and just uses it as a regular collar, of course the dog will learn to pull into it. The human has to actually control the dogs head in order to have *control* of the situation.
The same way it can learn to pull into any tool. The dog can learn to pull in choke chains, prongs, harnesses, martingales, and flat collars, if you just stick it on and do nothing with it. How many dogs have I seen pulling like a freight train on chokes and prongs, I cannot even begin to count. Head halters don't totally prevent pulling, unless the human uses it as it is meant to be used, and its proper use should not involved physical discomfort.
AgileGSD: Dogs seem to find headcollars pretty uncomfortable, based on the common reactions of struggling against the collar, fighting to take the collar off and change in personality while wearing the collar such as becoming noticeably "depressed" or as one poster here said "pouting".
Some dogs do, certainly. But some dogs don't, as well. And for those dogs it causes a complete depression in, I wouldn't use one either! For those dogs that do just find it annoying at first (the same way dogs find wearing clothes, or a vest, or a seatbelt, or even a puppy first learning to wear a collar), and once used to the idea totally ignore it or come to really love it. Some always find it a bit irritating, sure. But there's not a generalization that can be be made to say that *dogs feel this way about head halters*. Because then there are some dogs that take to it like a fish to water on the first try, and absolutely dont' care at all.