They're getting old.
It sounds silly, but House often forgets how long he's known them, how long they've all been standing shoulder to shoulder. Wilson's boyish good looks are slowly slipping away, but there's still that look; that oh-so-adorable smile, that charming demeanour. The look he uses to comfort the dying, to reassure the worried, to charm the women in his life – some talents never leave you. His eyes betray him; they're older, the pain of losing amber etched into them forever.
Cuddy ages gracefully, but the extra layer of makeup, the buttoned up shirts, hinted to House that she was ashamed. Aging showed weakness in her book, and she was as determined as ever to be strong. He watched her and Rachel grow together. Pride constantly etched across Cuddy's face as Rachel began her life, mixed emotions as she waved her daughter off to college. Her smile was still the most beautiful part of her. House noted how it still lit up the room when she walked in, how she managed to make everyone feel at ease. She was less quick to admonish him these days, or maybe he was calming down a little; he couldn't tell anymore.
Cameron, Chase and Foreman had remained at the hospital, forever advancing in their careers. The bonds they formed whilst studying under House tied them together forever; these bonds ran deeper, beyond mere friendship. They met regularly, catching up over drinks and expensive meals. Cameron and Chase were as happy as newlyweds, raising their own family, summers spent in Melbourne, Christmases spent in Chicago with Cameron's brother and his family. Their marriage had stood the test of time, their arguments and mistakes drawing them ever closer. Foreman had never married; both he and Thirteen felt that they "didn't need a piece of paper" to measure how happy, how devoted they were to one another. Thirteen had worked alongside foreman until she became too weak. He had nursed her throughout everything, maintaining as much of her dignity as possible, until she finally slipped away.
Cameron once told him that "not all change is bad", and as he watches them leave the hospital, he realises she was right. They've all changed; they've all matured. But maturity had brought them all closer together. He considered them all friends, not just 'colleagues'. He sees Cameron loop her arm through Chase's, two blonde haired children running merrily around, laughing delightedly. Foreman and Wilson leave the hospital together, laughing and joking, their separate tragedies helping them form a tight friendship. He feels Cuddy slip her hand through his, knotting their fingers together, squeezing his hand gently.
"Are you ready?" she asks, and he nods.
They leave the hospital for the last time, pausing briefly to look up at their home of 20 years. He glances at the woman stood to his side, where she seems to belong; tears are forming in her eyes, a small smile etched across her face. He tightens his hold on her hand; her smile widens, and she looks instantly younger. Images play in his minds eye, their first meeting back at college, days lazing in bed, furious arguments over treatments and ethics. This hospital had pushed them apart and brought them closer together.
Together, they walk away.