I'm intrigued by this quote from Series 3 Episode 12, The Sound of Drums (emphasis mine):
DOCTOR: See? It just shifts your perception a tiny little bit. Doesn't make us invisible, just unnoticed. Oh, I know what it's like. It's like, it's like when you fancy someone and they don't even know you exist. That's what it's like. Come on.
(Martha stares after him as he starts off, while Jack looks at her.)
JACK: You too, huh?
Obviously Jack has sussed Martha's infatuation with the Doctor, but I've never been entirely sure what the "too" refers to. It could mean "as well as Rose", but the first interpretation that came to my mind was "as well as himself". This is also the interpretation given at Wikipedia:
she received similar commiseration from Jack Harkness, who is also infatuated with him, in "The Sound of Drums".
But Wikipedia isn't a reliable source, and he could have been referring to Rose rather than himself.
Does Jack really have strong romantic feelings for the Doctor?
Since this is Jack Harkness we're talking about, flirtation doesn't count - he does that with everyone he meets. Kissing the Doctor goodbye in The Parting of the Ways also doesn't count, since he did the same to Rose at the same time.