The Doctor leaned against the wall, his arms folded across his chest. His leather jacket and wool jumper looked entirely out of place on this planet- they seemed to prefer lightweight, bright colored fabrics- but his expression did not invite others to comment on his fashion choices. He was frowning as he watched a couple sitting at a café across the street. The man- handsome Jack- laughed, and kissed his tablemate's hand. She was a local woman dressed in a sheer turquoise and purple chiton, and she blushed fuschia as Jack smiled at her.
The locals were more-or-less humanoid. A little taller, tending to be slenderer with long necks and pale lavender skin. This woman had short, pink hair (which had roughly the status of blonde hair, in this culture), and an elegant bone structure. Pretty, by this planet's standards, certainly. Jack always had an eye for pretty, no matter where or when he found himself.
She was leaning over the table, whispering something into Jack's ear. Now it was Jack's turn to blush, laughing. He raised an eyebrow at her suggestively, and said something back to her. The Doctor rolled his eyes.
But the truth was, as experienced as Jack might be, he could still be made to blush. Jack had that peculiar quality common to all the truly great lovers the Doctor'd met- Errol Flynn, Giacomo Casanova, Catherine the Second: he always bore his feelings openly with someone he loved.
Jack also had the other ability necessary to be a really legendary lover of men, women and others: he never slept with anyone he didn't love. Jack understood that love doesn't have to be forever to be true- that it can be honest, and still not require you to possess your lover or be possessed by them.
The Doctor admired that, much as it horrified him. Jack was strong enough to open his heart to the world, to love and to lose, and never to regret having done it. Most people love one or two people at a time. Jack seemed to have room in his heart for millions. He listened to his lovers, remembered them, and cared about them, even the ones he would meet once and never see again.
The Doctor himself was much less flexible. He had never been good about goodbyes, and it was too hard, too painful to let himself care about someone he'd only lose tomorrow. It was difficult enough to travel with these fragile, temporary humans- harder still to be alone, though.
A shadow passed over the Doctor's face, and he turned away, stopped watching Jack. For a moment, there was a desperate loneliness in his eyes.
Rose came up beside him, and it passed. "Well," she said, "Three orders of somethin' that's like chips. Sort of." She shrugged. "Where's Jack got to?"
The Doctor gestured at Jack with a thumb. "Enjoyin' the local color," he said. "Which, if you haven't noticed, is evidently pink an' lavender."
Rose snorted, popping one of the purple sort-of-chips into her mouth. "That's so like him," she said, rolling her eyes. "Always on the pull. Shall I run over and tell him we're ready to go?" she asked.
"Nah," the Doctor answered. "Leave him be. He's makin' a friend, and you can't have too many of those."