I made this observation at a writing-group session a couple of years ago, so I was happy to find my linguistic intuition supported with research. I used to have friends I texted several times a day, and I noticed that subtle changes such as this communicate even more with someone you text on a daily basis. At times, I could tell if my texting partner was angry or sad from the capitalization or the delay between responses.
Because text messaging is a conversation that involves a lot of back-and-forth, people add fillers as a way to mimic spoken language. We see this with the increased use of ellipses, which can invite the recipient to continue the conversation. The period is the opposite of that—a definitive stop that signals, as linguistics professor Mark Liberman has explained, “This is final, this is the end of the discussion.”There’s a reason using a period in a text message makes you sound angry