Cpt. Jack is the Doctor's brooding gay piece of fluff. I'm not making this up. This is cannon!

Cpt Jack: Hi!
Doctor: Stop flirting.
Capt Jack: I was just saying hello!
Doctor: For you that is flirting.

Capt Jack: You're just jealous.
Doctor: Nah. With you it's too much hard work.
Capt Jack: Yeah, but you know I'm worth the effort.
(The Doctor grins)

Also ... they kiss. Seriously.

In the episodes he's in the innuendo/flirting flies thick and fast.

You must watch this!!