Yellow Lab Lease Aggression

Question
My wife and I have a male neutered Yellow lab (Jake) that is 15 months old. We bought Jake from a breeder and he's lived with us in our apartment since he was 8 weeks old.
We know that Jake needs a lot of exercise so during the week we take him to a doggie day care. He's never had any problems at the day care and they tell us he's usually the one that goes over and greets all the dogs as they show up.
Recently Jake has become more and more aggressive while we are taking him on walks in the evenings and on the weekends. When we see a dog on the street, he usually lies down in a seemingly submissive pose. When the dog gets close, Jake will take a few sniffs of the dog, then the hair will go up on his back/neck and he'll snarl and lunge at the dog. He doesn't always do this,  but he seems to be doing it more and more.
I'm baffled because he seems genuinly excited to meet another dog, but when he gets close he seems to "snap" and become agressive. He's less likely to do this with dogs his own size, but he still will snap at them as well.
I try to be as relaxed as possible when meeting dogs as well as give him treats before we get close to the dog as well as after if he doesn't snap, but that doesn't seem to be making things any better.
We're thinking of enlisting the help of a dog trainer,  but any advice would be greatly appreciated!

Answer
Hi Nick, The problem with Doggie Daycare is that most of them do not take the dogs out for walks. Walking your dog at the heel exercises his mind and body. It takes more energy to heel than to just sniff around and pull you down the street. Jake is young and I understand that you do not have a lot of time. But to be fair to him, you must walk him at least 45 minutes a day, if not more. Many times fear can lead to aggression, and often people do not recognize the language the dog is showing prior to attack. When you see Jake take that down position, correct him right then, rather than wait for it to escalate. You could give him a check with the collar and leash, or quickly change directions to get him focused on you. Don't use the treats as bribes at this point. Seeking a trainer is great, but many trainers are not educated in canine behavior. So try to look for a canine behaviorist in your area to help you. It will take less time to fix the problem with a good behaviorist. Jake may not have problems at the day care because he is the dominate one in the pack. But when you are out with him, you must be the dominate pack leader, and Jake must be convinced that you are in charge. Thank you for writing. Leave me some feedback and follow up if necessary to let me know how things are going. Regards, Susan