It started with Papyrus, several months after the monsters moved up to the surface. He hadn't been feeling like himself; he was more sluggish than usual, didn't have much ambition and felt 'blah' all of the time. After going to Sans about it and asking for advice, the latter did some research and signed Papyrus up for a therapy session.
After the first session, Papyrus practically begged Sans to let him go back, and even suggested Sans go see the therapist on his own. After several months of Papyrus nagging him about it, Sans finally went. Afterwards he felt like a new skeleton.