Why we sign our emails "Thank you"

...the need for those sorts of rituals remains important, particularly in electronic communication where tone is hard to read. We end our communiques with “talk later,” “talk 2 u tomorrow,” or even simply “bye.” “Thanks” and “Thank you” have worked their way into this portion of the formula particularly in emails. More traditional valedictions have been replaced with “Thank you” so subtly that it’s now a common sign-off in this medium. But what does it mean? And why is it more acceptable than “Sincerely” or “Yours truly”?
It is in part be a reflection of our times. Email offers a speedier means of contact than an actual letter (and in some cases, a telephone), but that speed also means we’re sending more messages through this medium both for personal and professional reasons, and reading and responding to these messages requires a commitment of time. So it’s more important that the sender recognize the burden that they’ve placed on the recipient. In a time when letters took time to write, send, and respond to, it was important for the sender to attest to her reliability. Responses and actions were not so easy to take back. “Sincerely” and “Yours truly” which were meant to build trust between communicants. Credibility was an important determinant of whether a response would be issues. Today, as the web enables stranger to contact each other with little effort, credibility is less of a factor in determining responses (SPAM mail aside) when weighed against time.

From Scientific American.

I disagree with the end of the article, though, in which the author argues that affectionate closings are "vital to the continuation of the relationship."

The line between email and messaging (SMS, Facebook, Twitter DM's, WhatsApp, etc.) has blurred. In professional settings, you're taught that shorter emails are better, and that has removed one thing that differentiates email from messages. Since no one puts valedictions or even greetings in messages, they're starting to disappear from emails as well. Most of the email I receive no longer begins with a greeting like "Eugene", and most of them don't even end with a signature since it's clear from the From: line who the email is from.