I do the same thing. I wouldn't do it in early interactions with someone or with customers, but with colleagues and friends in conversations I often think that terse factual statements can be mis-interpreted, and I'd rather be thought of as unprofessional/frivilous rather than rude.
Also re-reading e-mails whilst mentally putting myself in the shoes of the recipient to see if I think it could be mis-intepreted helps avoid potential issues, I find.
Its a wise thing to do. I use it to simulate, how I would do in a face to face communication. So a smiley, where you would actually smile, to soften the blow, or to add humility when pointing somebody's wrong could be useful. Of course its use has to be limited, else it will look like a typical WhatsApp message emoticon train. :)
There are lots of small things, which if more people do, would lead to friendlier communications. E.g. replying with a greeting, to a mail which has greeting.
Also re-reading e-mails whilst mentally putting myself in the shoes of the recipient to see if I think it could be mis-intepreted helps avoid potential issues, I find.