I clearly remember this Bassett puppy, it was not enjoying any type of interaction at the park.  This would have caused quite a bit of stress.

Dealing with stress can be a day to day thing for humans; it can also be something that dogs have to deal with.  Depending on the dog and what their life looks like; they might have to deal with none, a little or a lot.  Like us, dogs are all different.  What one can handle, another may not be able to cope with.  Even with great socializing and life experience, some dogs have a difficult time dealing with certain things.  Take some dogs who are placed into the life of a show dog.  Many love it and do just fine; but there are others who just can't cope.  Lots of dogs are removed from the life of a show dog just because they hate it.  Many dogs are put through a great deal of stress; just because their of their owners desires. 

Is your dog happy?  Are they coping well with their life?  Do you think if they had the choice that they would live the life that they do?  I have always gone the way of "my dog's desire."  Meaning that what my dog shows me to be something that they love to do, we do or at least a version of.  That does not mean that our life revolves around that one activity but I try to incorporate it into our lives at least a couple of times a week.  Not having an outlet can cause stress as can forcing an unwanted behavior.

Was your dream to always have a Flyball dog?  Do you drag your dog to the event every week and just can't understand why they act off?  Do you have to go above and beyond to get through the learning curve?  Are you growing frustrated?  Well, it might all be due to stress.  Maybe your dog is stressed about having to go to Flyball and they would rather just not be there.  I've seen dogs in all sorts of performance venues who do not want to be there.  Many dogs can adjust and learn to deal but it could still be causing stress.  Stress is bad for us and bad for our dogs.  Stress is one of the worst things for our body. 

Whole Dog Journal - Easing Stress article. 

Stress can cause a dog to do all sorts of things that under calm and relaxed conditions, they might not.  I know that when I am highly stressed, it takes a toll on me and by the evening I'm spent.  Just imagine a dog who has to live through stress day in and day out.  Is your dog stressed?  What can you do to help stress?   If you are determined to participate in the activity that causes stress then you must help your dog out.  Small doses at a time and making it "all good."  That or figure out something to do other than the stressful trigger activity.  There are lots of things that you can do with your dog and there might be other things that are not stressful to them at all.  Stress is fairly easy to see but not easy to live with so ask yourself; are you stressing your dog out?