“Well then, Sherlock, back on the sauce?”
John and Mycroft’s way of saying, “because we care, you fuckwit!”
Notice in series 1, episode 1 John turned down money to feed Mycroft info on Sherlock, because he wasn’t interested. Now he realises Mycroft really is concerned he phones him without offer of payment, simply because they both care about Sherlock’s wellbeing.