My boyfriend has been playing this game lately, and I secretly changed the difficulty setting.
How do you think your boyfriend would react if he found out?
He'd be frustrated because he is going to struggle with the game now.
It isn't nice to change someone's settings and not tell them.  It would be frustrating for that person.
It's not a big deal. It's just a game.
It is just a game, but it's still rude to change the settings without your boyfriend knowing. Playing that game is something that your boyfriend seems to enjoy, and you ruined that for him. You should tell him what you did and apologize.