Sudden Agressions 1 year old male dane

Question
We have a 12 month old male mantle dane (not neutered) who is normally very well behaved.  He has attended two sets of obedience classes and received the canine good citizen certification.  So you can imagine my surprise yesterday when he was on a leash sniffing a male yellow lab and suddenly lunged aggressively and barked like they were going to fight.  An hour later, we went to the dog park that we have been to numerous times with no problems, and he did the same thing to a male 2 year old Newfoundland who was similar in size.  My dane Duke is approximately 135 pounds. I was shocked and disappointed, and really need to understand if he was just having a bad day or what we should do for next steps.  Thanks for your feedback.

Answer
These are the beginning signs that he is going through adolescence and reaching sexual maturity. I would suggest that you schedule to get him neutered as soon as possible. These are signs that his hormones are working in overload and if he cannot behave on leash and in a dog park, and remain intact, he needs to be neutered. As you have already taken the steps to ensure that he is well behaved via dog training classes and even his CGC certification, I cannot say that you have not done what you need to ensure that he remains social and well behaved. Even after he is neutered the behaviors may not go away immediately. It can take 2-3 months for the testosterone to fully make its way out of his system. You need to continue to take him out and correct him whenever this behavior arises. It only takes one time of not paying attention for a massive dog fight to break out. Some dogs can remain intact and never have behavioral problems, while others need a different course of action. If you are not actively participating in sanctioned conformation shows with your dog, the best and healthiest decision for him is to get him neutered before the problems become uncontrollable.