Why is the teammate bond often so strong?

Read this description of a detail from the movie i am legend. a scientist is the last man alive in a post-apocalyptic new york, where zombies have taken over and destroyed most of the population. about ten minutes into the film, the man thinks back to how the zombie creatures came to be, and how at one point he had a wife and child. which type of suspense technique did the screenwriter use? dramatic irony flashback foreshadowing special effects