Unfriendly and unapproachable faces - Metta Practices

I'm kind of curious, some people may have looked a little aloof and cold from the outside, and get commented on this too but deep down she/he isn't like this. She/he are probably just quiet and doesn't mingle around, don't think it's a wrong trait? How can metta help on this? And how can metta be practiced on situation like this? Thanks.