I wasn't 20 when I walked into Iliadis' bedroom. It was during one of the raucous parties that he threw with his brother at their bachelor-pad of a home in Athens; their father being the prime playboy then.
However, with weddings costing an AVERAGE of $196 per guest, not every couple feels like doling out "and Guest" to each and every one of their single friends, and sometimes take great pains to avoid the extra mouths to feed.
To be single at heart, I think, means that you see yourself as single. Your life may or may not include the occasional romantic relationship, but you don't aspire to live as part of a couple for the long term.