Headcanon that Sherlock messed with John’s phone one day and made it so that all his letters were stuck on capitals, so he can only text in all-caps. John was angry at first but Sherlock thought it was hilarious and refused to change it back, so now every time John has to send a text he lets out a resigned sigh and starts typing.
SH: Where are you?
JW: BUYING MILK. DO WE NEED EGGS?
SH: Why are you shouting?
JW: SHERLOCK I SWEAR I WILL KILL YOU