Because neither tsareena nor steenee are found in the wild, I’m going to assume that you caught yours as a bounsweet and raised her from there. If so, then the first thing to note is that the longer you’ve spent with a pokémon, the more it bonds with you. Additionally, for most pokémon, evolution is a rather big deal. For tamed pokémon, it represents the attainment of the highest amount of power they can possibly have, and because many pokémon strengthen their bonds with their human partners by fulfilling their evolutionary requirements (read: by battling), an evolution into a final stage—such as the evolution of a steenee to a tsareena—can sometimes be a symbol of the strongest possible bond between trainer and pokémon. In other words, your tsareena feels an especially strong bond with you, and as such, she wishes to express her loyalty.
The other thing to note is, of course, the fact that tsareena are regal by nature. As such, they take their duties extremely seriously and will fight to defend themselves and anyone under their care. This means that it’s actually perfectly natural for your tsareena to refuse to stand down when confronted by anything she doesn’t think is a friend right away and that it would actually mean something rather negative if she didn’t. That is to say, so long as your tsareena assumes the role of a bodyguard, this behavior means that she highly respects and values you and deems you worthy of protecting. If she chose to ignore you instead or didn’t take her relationship with you as seriously as she does, this would mean she considers you to be beneath her.
In short, this is a perfectly natural occurrence for tamed tsareena, in part because they tend to be with a trainer throughout their evolutionary process (and thus for a rather long time) and in part because it’s in their nature to defend their trainers with honor. I wouldn’t worry about your tsareena’s behavior until it gets in the way of your ability to perform day-to-day activities, such as interacting with people in your daily life (the postman, a barista, people at a grocery store, and so forth). If this happens, it would be prudent to train her to act only when there’s a clear and present threat to your well-being.