I would have thought it was obvious, the Platt has been on drugs since the girl with the dog moved into the Street briefly a few years back (if you remember).  He seems to be showing all the signs of drug addiction that we are warned about in the press, so this would be a perfect method to get over to youngsters the perils of dabbling in a 'harmless smoke' surely?

Why hasn't anyone questioned his mental stability anyway?  Poor Claire got bundled off quick enough!

Send him for rehab; boot camp and 'clean' him up before we all switch off with boredom about "what David does next"