Friendship

According to Temmosus, friendship grew with time. Those bonded by friendship were known as friends. (TV: The Daleks) The Eleventh Doctor claimed to never forget his friends. (PROSE: Hello!)

Friendship was rare among the Sevateem. (AUDIO: Chronoclasm) Daleks did not have friends. (TV: Eve of the Daleks)

Friendship was a value that was taught on Sesame Street. (PROSE: Writer's Notes)

The Doctor's friends
According to the Eleventh Doctor, he never forgot his friends. (PROSE: Hello!) Some of the individuals the Doctor counted as his friends, at various points, included: Sarah Jane Smith, (TV: The Seeds of Doom, The Masque of Mandragora) K9, (TV: Underworld, The Invasion of Time, The Pirate Planet) Bernice Summerfield, (AUDIO: The Unknown) Alistair Gordon Lethbridge-Stewart, (AUDIO: Minuet in Hell) Charley Pollard, (AUDIO: Embrace the Darkness) Donna Noble, (TV: The End of Time) Clara Oswald, (TV: Heaven Sent) Graham O'Brien, Ryan Sinclair, and Yasmin Khan. (TV: Resolution)

Charley Pollard and the Eighth Doctor expressed their love for each other as friends. (AUDIO: Neverland)

The Doctor also had many best friends. (TV: The Seeds of Doom, The Masque of Mandragora, Underworld, AUDIO: The Unknown, Quantum of Axos, etc.)

History
In 1963, the First Doctor described himself and Susan Foreman as being without friends after being cut off from their own planet. (TV: "An Unearthly Child")

In 100,000 BC, Ian Chesterton told Hur that he was her friend, but she did not understand the term, and thought that "Friend" was his name. (TV: An Unearthly Child)