Did dagon want tamriel as his own becasue he just wasnt supposed to have it? Or was it because he wanted to wipe out the ones who who were diffrent(mortals)?

He is the Prince of Destruction. His one and only goal is to destroy everything he can, especially Tamriel. As for why he doesn't attack or invade other continents is debatable. Perhaps he has, and there is no written document. However, I'm sure there is somebody here that can give you more accurate information.

I'm currently giving you watered down pieces of information:

Tamriel is special, perhaps for more reason than the (maybe) obvious one: the Convention took place there, i.e. Aka had Shor's Heart ripped out and shot down there. Tamriel is the "starry heart" of the world.

If you believe Mankar, Tamriel (if not Nirn) is almost Dagon's birthright, if it's true that Tamriel is Lorkhan's Daedric realm that is being "constrained" in a sense, by the "lying" Aedra. Basically, Dagon is essentially a hero that is "liberating" Tamriel.

By 'Dagons birthright' do you mean his birthright to destroy or to rule? What would he be liberating it from? And more importantly what would he be liberating it to? If he did conquer Tamriel wouldn't it be a wasteland of sorts afterward? If so, I would hardly call Dagon a hero "liberating" Tamriel. IMHO.

His birthright to destroy. Not only destroy the world "as we know it", but completely. This would make the mer (and men) "free" again (eg, return to their previously state as Ada) and that is part of why Mancar is following Dagon.

As for Dagon, he also wants to return to his original state as the "Leaper Demon King", but he is "cursed" to not be able to do so untill the world is destroyed.

Maybe Dagon likes wastelands, his daedric realms are pretty wastelandish.

http://www.imperial-library.info/obscure_text/aldudagga.shtml#1