Optimus did not know much about humans, human mannerisms or human physiology. He tried to understand (without coming off as patronizing, humans hate that) that humans were generally more fragile and could get hurt emotionally and physically more than cybertronians.

While not knowing much, he understood your recent behavior was not normal. You seemed more jumpy and on edge. Spacing away or clocking out of conversations when usually you'd be driving them. This concerned him, not only was this behavior detrimental to the autobot cause..he cared about you.

You were young, and he couldn't help but feel like a father of older brother to you. That was in his nature, and this drive, paternal or militant, made him walk right up to you first thing after a mission.