Jack is so much more fragile than most people would believe; they see him as the strong, tough hero, but they really have no idea. He's been broken so many times over his long life, in ways that just don’t heal, and Ianto’s painfully aware that the face Jack shows the rest of the world is in many ways a mask, hiding the truth about him.
That’s not to say that Jack isn’t strong, and tough, and a hero; he’s all those things and more, but he’s also human, with the same fears, needs, desires, vulnerabilities, and insecurities as anyone else.
He’s a flirt, it’s true; he enjoys brightening people’s day by making them feel good about themselves, and he relishes shocking people too, because he deplores 21st century attitudes towards love and sex. Most people seem to think all that flirting makes him shallow and unfeeling, but if anything, the opposite is true. Jack loves deeply, passionately, and without reserve, not just in the romantic sense, but also in the way he feels about friends and those he sees as family.
Even complete strangers matter to him so much that he willingly sacrifices himself for their sake, even though they’ll never know. It breaks his great heart each time he fails to save someone, and being forced to sacrifice a few to save the rest tears him up inside so much that if he weren’t immortal it might just kill him outright.
Ianto has no doubt that Jack loves him, even though the words have never been spoken between them; they aren’t needed because it’s clear in the way Jack holds him close night after night, and turns to him for comfort when the weight of the world drags him down. He knows Jack’s loved and lost many times in his long life, knows that each loss, whether through betrayal or death, chips away at that great heart a little more, and it scares him.
He isn’t naïve. Ianto is well aware that as a Torchwood agent he’s living on borrowed time; few make it as far as their early thirties, and reaching forty is almost unheard of. What will happen to Jack when he loses the one who comforts him through all his other losses? How many more times can Jack’s heart be broken and still heal enough to risk loving again?
Because Jack thrives on love, giving it perhaps even more than receiving it. Without it, even if he lives forever he’ll be dead inside, a mere shadow of the man Ianto knows and loves with all his purely mortal heart, and that doesn’t bear thinking about.