I understand why Jack didn't say I love you when Ianto died. What I don't get is why he said "Don't".

Jack made his way up from Archives, but stopped before he was noticed by his teammates. He heard his name mentioned and wanted to know what they were talking about.

"So how is everything going, you know, with you and Jack?" Gwen asked.

"Oh great, really well." The Welsh man replied smiling, thinking about his lover.

"Are you two like a couple now, or what?"

"Well, we do couple things. You know go out to dinner, to the cinema etcetera. But I don't think I'd use the word 'couple'."

"Oh, but you love him, right?"

Ianto stared at her and without thinking, gave a small nod. A huge grin came across Gwens face as Ianto confirmed what she already knew.

"Have you told him?"

"No. And honestly, I don't think I ever will. The only time I can see myself telling Jack I love him, is if I were dying in his arms."

The Welshman then walked away, not wanting to think about his death and finally having to say goodbye to Jack. Just as he got to his coffee station Jack came over and kissed him. Nothing more special than usual, but it meant so much more for both men.

"I love you" Ianto said, looking into his lovers eyes, slowly dying.

"Don't" was all Jack could manage, as he realised the moment had come.