Both series have the vibes interms of interactions between leads. HIDDEN LOVE features cute moments, giggles and light hearted conversation between leads through the time( both past story line and today). STARTUP THE ENGINE shows those moments in a limited way and only for the leads romantic relationship. The vibes, theme, jokes and feelings if the scenes are the same in both

These are stories with reincarnation of drama characters involved in a romantic relationship. In "See You in My 19th Life " the female lead is born again and searched for his previous life important person who becomes his romantic interest in current life. In "To all the Guys Who Loved Me" the leads had an unfortunate relation in previous life and a lot of incidents happened that male lead felt should be corrected. So in both dramas one of the leads used a past life information to built a better current life