When a sore is almost healed, and if a person rips or tears it open, it is way of preventing the healing process and further aggravating the pain. This phrase, metaphorically suggests, to revive or reopen a quarrel or enmity which was almost forgotten.
When someone gives you a pep talk it is to build you up to help you accomplish something. In sports a coach might give a player a pep talk before the game to bolster his confidence. At work the boss might give you a pep talk to get you to do a better job.