I often try not to sound formal when texting because it sounds strange, e.g. correct punctuation. I even go as so far to write extra letters to make it seem informal. For example, instead of typing "Hey," I would type "heyy."
In my opinion, when texting, it is hard to know what an individual actually meant, so that is why we use emoticons to express the emotion. For example, if you type "You suck," it would seem rather harsh, whereas saying "You suck :P," sounds less mean. The same is to be said with punctuation.