just started dog aggression

Question
I have a 5 and a half month Great Dane
he is well trained and has been to puppy 1 class. He can sit stay come down walk great on heel we plan on going for CGC and then through to CD with the hopes of a UD title.  It will be the first time for me to do this but i am so excited.  Now the problem started about about a month ago that he began growling, barking, hair raised, and showing his teeth at other dogs and about a week ago he started growling at people.  When he does we either give him a correction with a correction collar (we just bought a pinch collar and started using that) or grab his muzzle push down and say quiet.  Once he is corrected he just goes back to the unwanted behavior.   It doesn't stop.  I even put him on commands like down stay sit etc. but he will listen to me then looks back at them and begins growling again.  I think he is scared of them.  We have 4 dogs that he plays with all the time and if he is introduced to a new dog one on one it will take about 20 minutes before he calms down then he will play with them. I am worried about starting his puppy 2 class Jan 4 because of the way he is acting and I don't know if he will attack someone.  How do I stop this behavior and why did he start it? Please help me I know he can be great.

Answer
You don't want to correct for fear, and I think it may be a fear issue.  I'm not sure since I haven't seen the dog and I don't know how much obedience work he's really had yet... Whenever you correct it has to be over obedience, so if it's heel, you correct for breaking heel and lunging at another dog.  But you don't want the dog to think "whenever other dogs come around, I get a correction"  Do some private lessons with a trainer and work on building positive associations between your Dane and other dogs.  The obedience work will help things, too.  Be careful of who he plays with, but it may be good to let him play with a well-behaved and gentle dog.  It is not uncommon for a dog to live with other dogs at home, but be aggressive towards new ones that they meet.  The trainer should also be a behavior specialist.