one common effect of SSRIs is that they can dull emotions. This is helpful and can be a positive effect if you are very depressed or get stuck in anxious spiraling.
If you're very stable and happy emotionally, then that dulling might be unpleasant.
Beyond that, while SSRIs tend to have fewer side effects and be more safe than many other classes of anti-depressants, even SSRIs have very noticeable side effects. They are very easy to stomach if they make a dramatic difference in your mood and quality of life, but if you were already mentally well the side effects will be more impactful than whatever benefits you might get.
If you're very stable and happy emotionally, then that dulling might be unpleasant.
Beyond that, while SSRIs tend to have fewer side effects and be more safe than many other classes of anti-depressants, even SSRIs have very noticeable side effects. They are very easy to stomach if they make a dramatic difference in your mood and quality of life, but if you were already mentally well the side effects will be more impactful than whatever benefits you might get.