'Bring. It. On'.
It’s been Jack’s philosophy since long before the Doctor met him, and it’s what the Doctor loves most about him. His zest for life. 140 years living in the slow-lane haven’t dampened it one bit. Jack is just about the most alive person the Doctor has ever known.
Whatever life throws at him – and it’s thrown a lot – Jack meets it with a grin and open arms. And it never, ever gets him down.
And if he sometimes despairs, he doesn’t let it show.
The Doctor often has to fake his own cheeriness. Jack doesn’t need to.