To sum up the story, Yuuko ran out of time. She was 'created' because her existence is needed for "that day" and to achieve something. Watanuki is the reason she is here in the first place. She also explained numerous things (but not all was clear) to Watanuki before disappearing.

He was devastated. Yuuko is an important person to him. He wanted to make Yuuko's wish come true but couldn't. Watanuki made a decision. He will succeed the shop in hope of seeing Yuuko again... the time inside Watanuki's body 'stopped'. He stopped aging and shall remain the same youthful appearance for years to come. At the same time, while succeeding the shop, he cannot leave a single step (just like Moro and Maru). That is the price he pays for his wish.

Anyway, enjoy some of the images I uploaded. I like Watanuki better in his Yuuko-imitation style! And yes, he does behave a little like Yuuko since then - drinking sake, smoking her pipe... and his expression/ eyes is totally different.