easily approachable; warmly friendly
Yes, he looks quite stern and unfriendly, but if you strike up a conversation with him you'll find that he's really very affable.
ad (to)#fari (to speak) (Latin)
affable, amiable, personable
affection+able = affection to others affable = f + able, => since teachers are able to give 'F' grade to the students, students are always trying to maintain a warm and healthy relation with the teachers by being affable.