Personally, I think it's the height of hubris to say that dogs (or any animal) "can't love." Find me a scientist who can actually get into a dog's head and heart and understand what's going on in there...then get back to me on that one.
I agree with alwayshadpets. We don't speak the same language as dogs. And the emotions each species experience would probably be somewhat alien to the other, if we could actually feel them as they do. It's part of our human nature to project our own feelings onto those around us and assume they feel the same way we do. Especially when we love something deeply. Heck, sometimes we'll even want to believe that an inanimate object (such as a favored childhood teddy bear) can love us back.
So...do I think dogs can love us? Maybe not in the same sense that we understand it. But our word "love" comes closest to describing the devotion, loyalty, and affection they display quite clearly. The bond between humans and their dogs is powerful and it is real...no matter what you call it.