Nicolas DiDomizio, writing for Mic: 

'When using [a period] in a text message, it's perceived as overly formal,' Collister wrote. 'So when you end your text with a period, it can come across as insincere or awkward, just like using formal spoken language in a casual setting like a bar..

"Text [messages] and many other online forms of communication are intended to be brief, and adding a period which signals 'the end' is for many users a conscious choice and can communicate a message like, 'I'm really done talking about this,'" she said.