One great Kdrama to watch for a love story is 'Crash Landing on You'. It tells the story of a South Korean heiress who accidentally paraglides into North Korea and falls in love with a North Korean army officer. The chemistry between the lead characters is amazing, and the show beautifully depicts their journey of love across different cultures and political boundaries.
One common theme is unrequited love. For example, in many Kdramas, one character loves another deeply but that love is not returned immediately or at all. Another theme is separation due to external factors like family differences or social status. Just like in some dramas where a rich heir can't be with a poor girl because of family opposition.
They usually bring social issues to light. Like 'D.P.', it makes the public aware of the problems within the military that might have been overlooked before. This can lead to discussions and even changes in society.