The dictionary* definition of obsession is "a persistent disturbing preoccupation with an often unreasonable idea". In a dog's case an obsession might be tail-chasing, excessive licking (himself or something else), chasing shadows/reflections, spinning or fly-catching, to name just a few examples. It may be something that starts as an innocent or silly game but if it gets attention or some kind of positive outcome (in the dog's mind) it can become a habit which could then end up being harmful or dangerous.So it may start off with the dog doing something that is funny to watch. Everyone present laughs, perhaps even calling others in from another room to watch too, and getting the phone or camera out to record it. Someone may even learn how to trigger the behaviour and get the dog to do it on cue to show people. It only a bit of fun and only lasts a few minutes so what's the harm? Well, what's the dog going through and why is he doing it? In all likelihood, it's a sign of stress or a way of dealing with something which causes stress and the dog is probably in a state of panic. Look for one of the most common signs of stress - panting. Dogs only pant when very hot, physically tired, in pain or in distress. This funny little habit maybe doesn't seem quite so funny now does it?The question is how to break this habit and the cycle of panic that goes with it. Firstly, don't encourage the dog to do it and avoid whatever the trigger is, if known.If it's too late for that and the dog has already started, try ignoring him - no looking at him or speaking, possibly even walking away. If it's a way to get your attention and he realises he doesn't have it, that may be enough to stop him. Or, you may be able to distract him by calling his name and getting him to come to you. Praise and reward him for this but be careful not to be too generous with your reward and fuss, otherwise it could become an attention-seeking habit in itself.However, if your dog is too stressed to take any notice of you and has gone into some sort of daze, there is no point in trying to distract him. Instead, take him by the collar, or loop a slip lead over his head and just hold him calmly. If applicable, remove him from whatever's causing the problem eg if it's reflections on the wall, turn him to face the other way or position yourself so he can't see it. Don't talk to him, fuss him, cuddle him or stroke him. Just be a calm presence with your calm demeanour letting him know he's safe and there's nothing to worry about. You may need to sit down and get comfy for this as it might take a while for your dog to become calm. Don't bother trying to get him to sit or lie down, but if he chooses to of his own accord, that's great. Gradually and eventually, he should start to become calm and relaxed with his breathing returning to normal. You should find that the time this takes decreases each time you have to go through this process. The key is that you must be calm yourself - this is what your dog is going to learn from.It might be possible to do this when outside but don't be surprised if it doesn't help because there may just be too much going on for your dog to cope with.*The New Penguin English Dictionary 2000.