The entirety of Watanuki's interactions with Kotone.
Watanuki telling Himawari, who was convinced that after influencing his near-fatal accident with her uncontrollable Doom Magnet powers that he'd never want to see her again, that even if that's true, she makes him happy and that he considers meeting her to be one of the happiest moments of his life.
Related to the above, when Watanuki gives Himawari Tanpopo, explaining that the bird was born for the sake of being able to keep her company.
Yuuko validating Watanuki's existence as a real human boy, and him in turn telling her that he wants to be the one to grant her a wish for once and promising to do so.
Overlaps with Tear Jerker, but the fact that the last moments Watanuki and Yuuko spend in each others presence before Yuuko vanishes is them wishing for each others existence.
In the end, even though the result can only end up tragic, Doumeki always stayed loyal to Watanuki, carrying it to his descendents to make sure Watanuki wouldn't be alone again.