Jack watched Ianto leave. It took every ounce of willpower he had in him not to follow him. There was nothing he wanted more than to apologize and beg him for forgiveness…again. Jack was doing that a lot lately, apologizing to Ianto. He just couldn't bring himself to tell Ianto what he wanted to hear - to tell him what he deserved to hear. It wasn't that he didn't iwant/i to tell him. It certainly wasn't that he didn't feel it. He did; he didn't doubt it at all. He thought he showed it in many ways. He just couldn't say the words.
He hated himself for hurting Ianto. Three words were all it would take to make it better; to make Ianto happy. Three words that he knew were true. The fact that they were true was really where the problem was. Jack could say the words easily if he didn't mean them. If saying the words would get him what he wanted and then he could just walk away, it was easy. But saying it and meaning it was completely different. Saying it when he meant it made it real, made it more than just a games and sex, made it more painful when in the end he'd have to say goodbye.
And he always had to say goodbye. Everyone walked away or died eventually. Sometimes ihe/i walked away; he had to if they didn't. Eventually they'd notice they were getting older and he wasn't so he left before that happened. Walking away worked well for him; it meant he never saw the people he cared for wither and die. But Ianto was different. Ianto knew Jack wouldn't die. Ianto knew everything about him…or most of it anyway.
He didn't have to walk away from him, but that meant Jack would have to watch iIanto/i die. That was worse than walking away. Watching someone you love grow old and die while you never aged a day. He'd have to watch Ianto's resentment grow with every year. Watch his love turn into hatred because Jack wasn't aging. He'd let that happen once, a long time ago. He'd gotten married and had been honest with her, and then he'd spent the rest of her life watching her despise him more every day as she grew older and more frail and he stayed the same as the day she'd met him. He couldn't stand to see that look in Ianto's eyes.
So things would continue as they were. Ianto would be angry for a few days, but he'd find a way to make it up to him. A way to show him, without words, how important he was to him, and Ianto would forgive him. It was the way it had to be, for both of them. It made it easier later. Saying the words, or hearing them, would just make it harder.