Most problems can be dealt with, but take consistency, patience and repetition.
If a problem with agression suddenly arises without apparent cause, you should contact your vet in the first instance, in case there is a medical cause.
At the first sign of guarding food, toys, or furniture (curled lip, growling, snapping) you should seek help as you want it dealt with straightaway.
Some examples that you may need help with:
Puppy mouthing
Pulling on lead
Guarding toys, food or furniture
Lunging at other dogs (either in a friendly or aggressive way)
Anything else your dog can throw at you!

This is such a common problem that I have devoted a booklet to it and have devised a strategy to deal with the issues that arise, so that you can work with your dog to help the problem. Your dog may be reactive to other dogs due to aggression or exuberance.