Now, will dolphins ever communicate their desire to achieve greater legal consideration? Not unless we put some more effort into researching communication with dolphins (and even then, I strongly suspect not). Not really a problem though I think.
You miss my point. It's not a requirement for any individual, but it IS a requirement for members of a species in general. Most humans are functioning members of society. The ones that aren't, whether it's temporary or permanent are protected implicitly and explicitly through social contract and the laws we've created.
No dolphin will ever be a functioning member of our society, thus dolphins are not part of society, thus dolphins do not take on the responsibilities of being in a relationship with humans, thus dolphins are not entitled to parallel status.
Is it? Considering we have never before in recorded history extended such consideration to another species, it seems unlikely that there are existing standards we can look to.
There are many things you can observe "most" humans doing, but that does not mean those things are all prerequisites for special legal and ethical consideration.
How can you possibly make that sort of generalization when there is only one species that is commonly accepted as having rights?
We can do whatever we want. We can give citizenship rights to gummy bears because they look like real bears and bears have two arms and two legs just like people. It would be illogical and counterproductive to any real advancement for society, though.
Exhibiting some rudimentary social behaviors doesn't qualify as accepting the responsibilities along with the rights accorded with status.