Someone once told Jack that you can't regret anything because at the time, it was what you wanted.
Jack thinks this is total and utter bollocks. He has plenty of regrets. Living as long as he has, it's a fact of existence that he would do things he shouldn't (lots of those) and not do things he should. It was the latter type of regrets that really kept him up at night.
The Doctor for one. Jack regrets that he never did buy him that drink. The Doctor was brilliant and brave and Jack found few things sexier than that. Of course, the Doctor was also quite easy on the eyes and that didn't hurt anything. What stopped Jack from giving up the careless flirting and pursuing the Doctor was Rose.
Rose was another regret. He always assumed she would be there. When he finally found the Doctor, Rose would be there standing beside him. She'd rush to give him a hug and the Doctor would stand by and say something sarcastic before letting himself be pulled into the embrace. And this time, Jack would make his intentions known. He'd tell them that they had changed him for the better and that he loved them.
But he never got the chance. That's a lie, he had plenty of chances, he just never took them. Jack always thought there would be more. And all he had now were memories and the quick, stolen kisses he finally had with each of them.
Among all the things, his Doctor and his Rose taught him the fact that regret could hurt worse than any heartbreak was one of the most important.
So, when he catches Ianto Jones looking at him with more than just a friendly interest, he makes a decision. Jack felt something deep inside himself stir when Ianto looked at him or smiled at him, or just stood by looking so good in a suit it restored Jack's faith in British menswear. He didn't feel these stirrings very often, lust was a completely different emotion altogether. So, when Ianto took the plunge and moved their casual, but intense flirting to the next logical step, Jack didn't stop from turning himself over to the Welshman. Soon, their casual fling became less about release and more about companionship and care. No matter how badly this ended, no matter how hurt he would be when Ianto died and Jack went on, he resolved then and there to spend as much time as possible showing and telling the beautiful young man how much he meant to Jack.
He likes to think Rose and the Doctor would be proud.