Am I treating her for a negative reaction or am I showing her how fun it is to see another dog?

Based on the scenario you described, your consequence (the food rewards) are not capable of targeting any particular behavior or reaction, therefore the efficacy of your method is too, hit and miss. The problem with your approach is that you have no control over what dogs will enter your dog's environment and at what distance. Instead, I would prevent the dog from rehearsing the behavior by either keeping her in a crate or use a calming cap.

Only until you can control the dog's environment can you answer the question, is the behavior respondent or operant? 90% of the time (guesstimate) the behavior is respondent, and as long as your dog is accepting the food rewards can you then test your method. Otherwise, no association is being made and you're just allowing the dog to rehearse the behavior and you're rewarding behavior randomly.

 

I have to agree with curb.

What I have done when working with dog-aggressive dogs is to first determine the dog's comfort zone (the distance before he reacts), and to keep him within that zone. If you continue to expose him to other dogs to where he reacts, you are actually reinforcing the undesired behavior. So, it won't get better, it may get even worse, and, it will be harder to change/stop.

Desensitization and counterconditioning work well, but you need to leave the other dog(s) at home, only working with the dog who is reactive, so you can control things. When on a walk and you see an approaching dog, turn in a wide arc to the left, which will place your body between the oncoming dog and your dog. Get and keep your dog's attention (i.e., a stream of upbeat "happy talk"), and calmly keep on walking.

Go hang out in the parking lot of your local pet store during the hours when customers and their dogs are likely to be frequenting the store. Stay far enough away from the entrance, so you control the distance between your dog and the other dog(s) coming and going. I bring really yummy treats, like braunsweiger or garlic chicken, or summer sausage, in tiny pieces. I treat for calm behavior during the coming and going of other dogs. In this scenerio, I have included goofy behavior on my part (clapping hands, dancing around, pointing in the direction of the dog and saying things like, look! doggie, lookit the doggie! YAY doggie! Good Boy)! It's been my experience that dogs like it when I act like an idiot! LOL It DOES get the dog's attention! Over time, I gradually close the distance, working our way up to the door, and for meet and greets.

 

Actually, the way you're doing it CAN work- if you do it right. And you're not quite doing it right. But you're close. There's a technique called open bar - stressor appears, treat appears, stressor disappears, and treat goes along too and the dog learns to look to the handler for their treat at the first sign of stress. It DOES need to be done at a distance and carefully- you want the dog aware of the stressor, but not over threshold (aware of the item that makes them react but NOT so close a distance at whic hthey'll react to it) and the biggest mistake most people make is moving closer TOO quickly. The other big mistake I think you're making is that your timing just isn't good.

The booki "Control Unleashed" might have a lot of useful information for you.