Ianto loves with a fierce single-mindedness that terrifies Jack.  He's died a hundred times, been to the end of the universe and back, but to be so responsible for another person...well, that's a bit much.

He should have realised it, he knows, should have understood that a man who would keep his girlfriend alive and hidden as a cyberwoman in the basement of Torchwood for months could never be casual.  But Jack has only recently come to understand that there are two different ways that people love: some, like Jack, love widely (though he's wider than most) and easily, shagging anyone in their path.  They can carry on a relationship, even care deeply, but in the end they let go easily enough.  Then there are those like Ianto.  He pours all his attention, his energy, his devotion into one person.  He'd die for Jack, even knowing Jack can't die.  And it's killing him, second by second, that Jack does not love him the same way.

Jack hates it.  No, if he's being truly honest - and there's no reason not to be - he knows that he should hate it, should stop it, but in his own selfish way he enjoys it.  He did not ask for it, never wished for it, but to know that Ianto loves him so completely - it's flattering.  In a terrifying way.  In the Year That Never Happened, he had time to think, too much time, and he realised that while he cares about Ianto a great deal, he will never feel the same intensity - he's not capable of it - and so it was necessary to distance himself.  Set boundaries, that sort of thing.  He's not sure where the date came from.  It seemed like a good idea at the time, a way to make clear that they were just - just dating, just shagging, just something.  He'd hoped to keep Ianto from falling in love with him, but it was too late.

Jack doesn't see any way out of it, either.  Not without dying - that was the only way Ianto would abandon Lisa, after all - and that's off the menu.

He's not sure he wants out, either.