Sherlock has never had anyone there for him why would he call for help when the only one who would come running would be John? At these points in the story he doesn't understand yet that Lestrade and Molly love him and so John is all he CAN call out.
"Why are you staring at me?" "I just want to make sure you like your coffee." "...You didn't put anythi---" "No." "Are you sure?" "Yes." "Are you lying to me?" "........Of course not. Tell me, are you seeing anything strange?" "......Sherlock......."