They knew they should have seen the signs. They felt that they should have been able to stop Trav. Kate and Buck realize that they have to go on with their lives even though they have lost a dear friend. The theme of the story is that there are little things in life that are meaningful to someone, yet they have absolutely no purpose to another. One person's treasure can be another person's trash.
Life is strange.
A person has to live for the great times in order to survive.