Love. It was a strange thing. Jack had never thought he'd feel it. But he had. Not many times but it was enough. There was Estelle. Lovely sweet Estelle. She thought he was just a descendant of the one she new so long ago. Seeing her again and then watching her die. It was the first in a long time that he'd properly cried.
Then there was the Doctor and Rose. Two people. One human one Time Lord. They came into his life and seeing how they were he'd at once wanted to change what he had been. He wanted leave the past behind, his conning behind and start a new future with them. But then they were snatched from him again. He died came back to life and he still didn't no why. But they had gone, leaving him behind, thinking he was dead and he never saw them again.
Then it was his team. Ianto in particular. He knew that when he was here on earth, the reason he always came back was because of them. Because they needed him. And it may not be the most important reason but it was very important to him. And he would keep coming back for them. Even after they were long gone.