Tulkas Purpouse

I agree Tulkas willingly volunteered to come to Arda and aid the Valar:

But in the midst of the war a spirit of great strength and hardihood came to the aid of the Valar, hearing in the far heaven that there was a battle in the Little Kingdom...So came Tulkas the Strong...

It seems Tulkas couldn't resist; he was like a fresh calvary coming to the rescue in the nick of time, for the Valar were weary of their labors and battles with Melkor. It seems he enjoyed residing in Arda too, for he took a spouse, Nessa.