if you're looking at it from this angle I guess you're right, but IMO that's not quite what the episode is about - for me it's about love, what it can do to you and what you're prepared to do for it, not just Ianto, but Lisa killing the pizza girl and transplanting her brain to become human again out of some memory of love, when the very act proves how far she's removed from being human. The tragedy of it, and the suffering.

Equating this with addiction is certainly a possible interpretation, but a bit too negative for me, and I don't think canon really supports this reading, either. TW doesn't idealise love, people get hurt by it all the time, but as bleak and existentialist as the show often is in other respects, it's never been cynical about that. John's love for Jack redeems even him to some extent in the end. I think there's a reason CW doesn't end with a clear message or easy moral, or even Ianto admitting that what he did was wrong or that he shouldn't have done it.

And I think in the end Jack understands Ianto's reasons, or else he wouldn't have been allowed to stay.