Jack does not, and cannot, love Ianto.
He realises this shortly after one of his many deaths, curled in post-coital bliss with a sleeping, or close to it, Ianto. Ianto had uttered the "L" word while they were together that night, and although on any other night Jack would have tried to rationalise it as him touching the right spot inside of Ianto or holding him in the right way, but he knows that it's true. Ianto does love him. Probably always has, under thinly veiled comments about his coat and relentless stalking and perfectly made coffee. Jack would like to love Ianto. It would have been easier, if not on Jack then at least on Ianto, but Jack cannot summon the stirrings of love that Ianto so craves. Ianto is attractive, yes, and can be charming at times, and Jack has to admit that he enjoys desecrating every inch of his perfectly-pressed suits, but while there is lust and attraction, he doesn't love Ianto. It's probably for the best, anyway--everyone Jack has ever loved has been lost, and he knows he doesn't want to lose Ianto; almost losing him is hard enough. In this way, he can justify and rationalise not loving Ianto, but even a logical justification can't stop his guilt.
Shifting them carefully so that Ianto is facing him without his sleep being disrupted, Jack studies his face; his round cheeks and high forehead, his impossibly long eyelashes and the way he looks so young when he's sleeping, younger than he normally does, almost like a child. Ianto sighs in his sleep and cuddles close to Jack, resting his head on his shoulder and clenching his fist above Jack's heart. Jack holds him there protectively for several minutes, but still nothing, and by now the post-coital glow has worn off and Jack is left with nothing but guilt. With a sigh, he disentangles Ianto--who whimpers in his sleep, causing Jack to freeze, before drifting back off, his hand now flung over his forehead--from him and stands up, reaching for his trousers.
Before he leaves, he tucks the covers up around Ianto's neck and kisses his forehead, brushing the stray lock of hair that falls there off. He makes sure that there's cold water by the bed in case Ianto has a nightmare; it happens sometimes, especially after Jack has died, and Jack knows that if he has a drink of water he'll go back to sleep, then dresses quickly and leaves without looking back.
He doesn't even make it out of the apartment building before the guilt seizes him. He recalls Ianto's face when he came back, the flash of unabashed love and joy and then the quick veil of tentativeness before he stepped forward and buried his face in Jack's collar. It's the same expression his face takes on every time that Jack comes back, as though every time, he missed Jack so much that it nearly killed him, which, if the others are to be believed, isn't exactly an inaccurate assessment. It's the way Ianto loves him that makes Jack wish he could love Ianto, too--without any expectation of love in return, Ianto loves him, relentlessly, unconditionally, and without the expectation of any love in return. As far as Jack knows, as long as Jack continues to shag him and as long as he still has his job to come to every morning, Ianto would be content with the arrangement they have now for the rest of his life.
Which, of course, is true.
Before he has time to change his mind, Jack turns and dashes back upstairs to Ianto's flat, letting himself back in, thanking God for his spare key, and moving quickly toward the bedroom, drawing Ianto back up into his arms and cuddling him. He does not, and cannot, love Ianto, and he knows that, but he also thinks it can't hurt to pretend, just for the night.