I agree - my dog is always on the lead but used to go happily off leash in dog exercise areas.....until she and another small dog were hounded by 5 other dogs - the dogs were also jumping up at myself and the other dogs owner. We couldn't get away and it was very distressing. The owners of the 5 dogs (who were standing around chatting, didn't give a hoot and in fact laughed at us).
She is now quick to snap at any dog that approaches her so it makes taking her out stressful - we can no longer go to dog exercise areas and even on walks it's difficult when other dogs are off leash and want to play with her. It's annoying as I took the time to do obedience training with her and one event when she was 6 has had a lasting impact (she's now 10).