Jose, The expression “part of” is used idiomatically without an article 1), but it can also take an article 2) and have essentially the same meaning. In other words it can be treated as uncountable because the article is not required, but not because you can’t count the parts 1) [Undeclared aggression, war, hypocrisy, chicanery, anarchy and impending immolation]… these things taken together…… ARE PART of our daily lives. -Thinking is part of my daily routine. 2) [Undeclared aggression, war, hypocrisy, chicanery, anarchy and impending immolation]… these things taken together…… ARE A PART of our daily lives. Thinking is a part of my daily routine. With the plural the meaning is also essentially the same. Undeclared aggression, war, hypocrisy, chicanery, anarchy and impending immolation ARE PARTS of our daily lives. -Thinking is one of the parts of my daily routine. De rien!

In the sentences you provided, the word "part" is singular while "parts" is plural. Because in the last sentence it lists each individually, you use "part" while life becomes lives(plural). In doing so you attribute each word to the lives being talked about. If you want to say "aggression, war, hypocrisy ARE PARTS of our lives", you then need to continue the sentence because if not it seems incomplete. for example you can say:aggression and hypocrisy ARE PARTS of our makeup that needs to be dealt with indefinitely. i hope this helps and does not confuse you.