For Agmaer, the reason is simple. He heard about what’s going on, the vampires, the Dawnguard and all, and he wanted to help.
We reached the foot of the hill leading up to the fort and Agmaer bides J’Zhirr farewell. “I guess this is it,” he says as he starts up the road alone. “Wish me luck.”
J’Zhirr hangs back for a while and waits until Agmaer climbs up the front steps. This one is in no hurry. Let Agmaer savor the moment.