The Doctor had adopted you (basically) and he acts as your father, despite you guys basically being the same age. You were on an adventure, a chill one at that. And you were sitting next to him. He decided to make a pun, which he’s definitely gotten to dad joke level of bad. (Which is good bad) As soon as he makes the pun you laugh and groan “DAD REALLY?” everyone except you and him are confused. They didn’t realize the fatherly dynamic he had with you. While you two are laughing your lungs out