[806mtson]

Note: This post contains spoilers for Episode 24
Man, from the looks of things, I think that Primitive Dragon has at least at some level of sentience and is at least somewhat protective of Touma. When it sensed Touma was in danger of being taken down by Legeiel, it decided to swoop in for the rescue and power up Touma into Primitive Dragon Brave Dragon.

I mean, Primitive Dragon could have easily decided to possess another Swordsman. Like one of the Sword of Logos group. It chose to save Touma and give Touma its power. Even though it is controlling him, I think that Primitive Dragon is at least somewhat protective of Touma.

Looking at the Wiki, this is part of Primitive Dragon's Description:

Quote:

According to Master Logos, the book contains a lonesome soul whose sorrow is incompehesible [sic], which causes it to go rampage and destroy everything. He also claims that it will bring glory to his organization someday.

Given that he said that "the book contained a lonesome soul whose sorrow is incomprehe[n]sible", it saw itself in Touma due to him being alone. His sister is missing. His comrades abandoned him due to Reika. And his last link to his childhood in the form of Kento Fukamiya/Espada is gone. It saw itself in Touma. I think the reason why Elemental Dragon can counter the Berserker effects of Primitive Dragon is that it shows that Primitive Dragon is not alone. Look at the 2 Books for the Form. The Claw of Primitive Dragon interlocks with the Claw of Elemental Dragon. It gives the illusion that the Two Dragons are Holding Hands. In fact, the transformation announcement even says "Shake hands!"
Though, one of the books found in another place seems to be glowing, but it's only shown glowing when Touma transforms into Primitive Dragon. Maybe that is Elemental Dragon. Maybe it requires the presence of Touma in Primitive Dragon Form to finally transform into the Elemental Dragon Wonder Ride Book.