If you take Love in the Big City's episode-6 relationship arc; combine it with episode 8's nostalgic, melancholic, and temporally disjointed remembrance of times past (and idealized expectations pulled back down to reality) on vacation in a foreign land; intensify the moody sense of loneliness and dislocation; make the humor a bit more subtle; keep the attention to grittier dimensions of gay life, the more explicit elements of gay sex, and the questionable maturity of the very flawed (i.e. human) protagonist; and you film the story in an art-house style; then you might get something like Happy Together.

Both works depict challenges with being gay in South Korean society as well as experiences of familial relationships for gay people, and both works do so with a rather literary approach to writing (focusing on arranging words in a beautiful and profound way); a somewhat melancholic but also hopeful ending and overall emotional tone; and an achingly beautiful instrumental score for the soundtrack. The writing similarities are strongest with episodes 3-4 of "Love in the Big City", which (like "So Long, See You Tomorrow") makes heavy use of universe-related imagery.

Both films examine questions about the meaning of family and the impact of money, through stories about a queer relationship across class differences in which one partner dies, leaving behind a partner and family members to grieve their passing.

Both films take a very figurative, "artsy", vibes-based approach to their narratives and themes, and both rely on subtle allusions to history. Both films place queerness (as a transgressive way of doing sexuality) in relation to migration and shadows cast by past conflicts, and both films do interesting things with temporality and spatiality.

In both films, the child of a queer parent unearths their parent's queer past by leaving home and, in doing so, changes the relationships around them in the direction of healing.

The overall plots of these two films have some interesting parallels and shared themes at a high level (including a story which bridges multiple decades of experience, a relationship between two high-schoolers which changes their relationship with a hostile world, and a separation which interrupts that relationship for a long time), and both locate their story in regionally-specific contexts. So Long, See You Tomorrow has less of the dramatic teenage angst of being a queer teen in a past decade (opting instead for a quieter form of teenage angst), a more meditative approach to its story, and a more hopeful and less bittersweet tone in its ending.