You weren't the most well observant man of how your body responded, too focused on your job to really care. But the Astartes you were serf to noticed.

Offering what is needed the Astartes took it before giving you a pat on the head, you didn't even notice but at the pat like a dog your leg moved. The Astartes found it amusing, especially your obviousness to it, he just had to do it a few more times.

So you stood there, taking a few pats on your head. Unsure as what was so funny.