I would argue that having a camper, or campers that you get along with better than some others is an inevitable part of being a camp counselor, along with homesickness and yet another squabble over the latest fad, including Yu-Gi-Oh and Pokémon. This is due to what happens in other social situations: sometimes different people gel better than others, and counseling is no different.
Being inevitable, there is nothing wrong with this; you can have a really good summer if a couple of kids in your bunk are on your wavelength. The issue only arises if this is to the detriment of the other campers. The best way to avoid this is probably to be aware of it. If you are conscious of favoritism, this means that you can take steps to avoid it. You could spend time concentrating on the other campers as well, for example. Or do a group activity where everyone can get involved equally.
The other side of this coin, I think is to realize that you can’t be all things to every child: you can’t have that amazing relationship that completely turns a child’s summer around with every child. Some children won’t connect with you as well as others. The thing to bear in mind, is that you can still have fun with them, and still be a part of their summer, even if you don’t feel you <i>are</i> their summer.