During the time before the Mandalorian Wars, when Meetra was teaching a young Mical, she would often show her affection/encouragement by petting his head. This eventually became a habit which she uses to relax and comfort him.
Years later, they meet again. Meetra is unsure if this man is the same youngling she had taught all those years ago, and so, going through the same motions again, all doubt was erased when she saw the look on his face.
She still does this from time to time, forgetting that he’s a man already who is taller than her :^^)