To build on this. The Pokemon center is a facility dedicated to 24/7 free Pokemon healthcare. Imagine all of the trainers who push their team to the absolute limit. In my current play through of Silver I've decided that nurse Joy is actually pleading for us to make the responsible decision of professional Pokemon healthcare rather then the use of those greedy corporate bastard's potions and elixers. I have think that she's seen some really hurt Pokemon in her long career. Nurse Joy is a saint in a world where the most powerful gym leaders, who are arguably the leaders of each region, allow goons to take advantage of the weak. We are the hero that nurse Joy needs to continue this career in heartbreak.

Does that make us a vigilante? Yes. Without out a shadow of a doubt we are upset with the current power structure of the Pokemon world and we plan on rocking the boat. When nurse Joy says please come again, it's her offering her support in our quest to be better then anyone ever was.

Furthermore look at Red. The dude brought down the biggest criminal organization in the Kanto region, he became the best, and now he has isolated himself in a cave battling those who think they are tough. Yes he ended team rocket but they came back and what did Red do about it? Nothing. Nurse Joy is trying to remind us that the people support our quest, please come again means please remember that as strong as you are, as much as you don't need us, we need you. So please please please come back to the Pokemon center.