Each blank provided here could have its own words that convey the meaning desired in each case. To find a word that applies in both cases, let's break down the example sentences to see what is really being declared.
It is difficult to write a paper about the meaning of life because the topic is so
Political discussions at the Thanksgiving table are
The expressions we're left with common to both example sentences are focused on the aspects of these topics that are dependent on the perspective of an individual. The real reason a Thanksgiving dinner discussion won't reach a conclusion is because everyone's perspective is different, and the reason writing about the meaning of life is difficult is because finding meaning in life is a task unique to every individual.
Based on this, I suggest subjective, citing OED definition 4.a.
Of, relating to, or proceeding from an individual's thoughts, views, etc.; derived from or expressing a person's individuality or idiosyncrasy; not impartial or literal; personal, individual.