This is context. It explains a common belief, setting up the author’s argument that this belief is wrong. While many people think personal conflicts are unavoidable, the main conclusion is the author’s argument against this belief: that personal conflicts aren't inevitable.

This is a premise. It supports the author’s conclusion that personal conflicts aren't inevitable but arise because people act irrationally. The statement that people “find it easier to ascribe bad qualities than good ones” gives an example of this irrational behavior.

This is a premise. The idea that it's irrational to let one instance make one certain a friend is unreliable backs up the author’s claim that humans act irrationally. This, in turn, supports the main argument that personal conflicts happen because of this irrational behavior.

This accurately rephrases the main conclusion. The author argues that personal conflicts are not inevitable, even though many people think they are, and concludes that the belief in their inevitability is "just not so."

This is a premise. It supports the conclusion that personal conflicts aren't inevitable but arise because people act irrationally. The idea that it takes years to trust a friend but only one mistake to doubt that friend provides an example of people’s irrational behavior.