Smown is a combination of the words "smile" and "frown" and describes a facial expression whereby a smile is quickly replaced by a weak frown. This is sometimes accompanied by a soft sigh. As a "smown" contains aspects of both smiling and frowning it is better at expressing more complex emotions. If one witnesses a smown in another it possible that the "smowner" is feeling so much joy and/or pleasure that he/she is touched and feels emotional thus triggering a proto-frown.
*As "smown" is combination of two words and their definitions it is categorized as a portmanteau word.