I had always thought this was done with someone you fancy. I have moved to a new city and had a few flings with guys who did this at the end of texts. Though one guy did this even as he was clearly closing himself off from me, which was confusing. I ask because there is a guy I know a bit and, the last time I saw him, his behaviour made me think there is a least the potential that he is interested in me. I added him to a WhatsApp group I am in, and when he replied to me on it , he put an 'x' at the end of the text. Is this a polite social convention or could it be a signal he is interested? He's probably later 40's in age.