Spoilers to AtS S5 - Soul Purpose
Fighting the good fight - he remembers the glee he felt when he discovered he could still hurt demons. He remembers being the strong warrior whose help was needed, if not exactly wanted. He remembers working with a team, as incompetent as they were, and getting a job done. He remembers working with her, their moves matching, covering each others blind spots. He remembers the adrenalin of a good fight, fists and fangs and nothing else - though a sword and stake were also handy. Here he is fighting the good fight not for the fun, but to keep boredom at bay.