In "The Doctor’s Wife" the living consciousness of the TARDIS was locked in a human body meaning the Doctor could finally converse with the "old girl" also known as Idris. When the Doctor first touched the TARDIS console he said it was the most beautiful thing he’d ever known. The TARDIS claimed she had chosen him on Gallifrey. The TARDIS needed a Time Lord to see the world and chose the Doctor because he was"‘the only one mad enough."
They make the best team in the universe. Ever.