Similar feel with heartwarming moments, family & sibling relationships, reconciliation and sacrifices.

The premise is so similar that one may think Ugly Alert is inspired from Happy together. There are some differences in story but a lot more similarities. The OSTs are also good in both dramas.

Similarities
- Both dramas revolve around family, sacrifice and regrets especially the relationship between parents and children + spouses.
- Both dramas have a warm feeling to it.
- Both dramas are have a parent as the protagonist - father in 18 again, mother in Hi Bye Mama.
- There is no real villain
- Both dramas highlight some social issues. 18 again: women empowerment, teen pregnancy, bias in society, etc. Hi Bye Mama: bias against step moms.

Differences:
- While 18 again revolves around love and hurt on a daily basis within a family, hi bye mama is themed around the pain when one is lost to death.
- 18 again is a lot more realistic and relatable and has only one fantasy element to it while Hi Bye Mama has more fantasy elements to it compared to 18 again
- 18 again's driving regret revolves around what could have been done if family wasn't there while Hi bye mama's regret revolves around what could be done for the family.