Those three words echoed in his mind over and over. I love you. Ianto loved him. In all the years that Jack had been alive only a selected few actually said those words to him and Jack longed to say them back. He longed to tell Ianto how he felt. He did love him, but knowing that he would outlive Ianto had caused him to suppress his feelings. Of course the odd kiss and part time shag wouldn't make up for how he felt. Yet he still wouldn't say it.
Those three little words had an echoing effect on Jack's life. One that reminded him every day of what he didn't do. Yet deep down Jack knew it wasn't his fault.