Your front door is different than the rest of your house in that anyone can come knock on the door, like a mail man.
"The Court has acknowledged that a doorbell or knocker is typically treated as an invitation, or license, to the public to approach the front door of the home to deliver mail, sell goods, solicit for charities, etc. This license extends to the police, who have the right to try engaging a home's occupant in a "knock and talk" for the purpose of gathering evidence without a warrant. However, they cannot bring a drug detection dog to sniff at the front door of a home without either a warrant or consent of the homeowner or resident.[102]"