I cannot say it better than what animalfreedom.org has already stated.  This is only a very small excerpt from their site.  If interested in reading further, please click the link above.

Animal Rights & The Theory Behind It

What motivations for animal rights are relevant?

Which concept of animal rights is the best? Is it respect, freedom, intrinsic value, love, compassion, equality or something else? What should be the basis for animal rights?

It is in the nature of animals to assert themselves in the animal world but this in itself has nothing to do with having rights. At a certain point in time man conceived the notion of 'rights' and it is man alone that employs such a concept.

Why then do we speak of animal rights? The answer is quite simple: animal rights are meant to set limits to human behavior. If we fail to set clear legal limits to human behavior in relation to animals, it will be impossible to initiate legal proceedings against those who exceed these limits. Animals are vulnerable, defenseless and completely in man's power. Persons who disregard the well being of animals should be brought to court and be held accountable for violating animal rights.

To continue reading the article click the link above.