Helmet bumps, a way you've shown affection to Doomguy. Just a light tap at his helmet with yours, an act of affection. Originally Doomguy didn't get it, but he had nothing against it allowing you to do so. But now figuring out it's an act of affection, Doomguy tries to do it back.

Course being a tad aggressive it's more like a headbutt. Now laying on the ground taken back by the hit Doomguy is profusely trying to apologize. Picking you up of the ground, worriedly.