[He literally just said he can't explain it dang.]

When... [hm.] There's a point where... you reach the crossroads. And you.. i think you have to be on the verge of death. That plays some part, but you have to choose. To become a full Avatar or... stay more human. And... somewhere in that, I.. chose it. Before I woke up. Oliver is the... Avatar of Death. So ....

I may be entirely wrong. I'm not sure how it works exactly. I just know I unconsciously made the choice. I thought it'd be better than dying.