And this is the perfect opportunity for the Doctor to try to influence Will just a bit. Because it becomes so important to him later in life and he needs to know there's at least one person out there whose life he didn't mess up.

"When you get back to me. The me from your time. Do me a favour and keep living your life. Travel with me, if I give you the chance, but keep up your own life outside of the TARDIS too."