When you use anabolic steroids your body stops producing testosterone of its own.
When your body does not produce testosterone your body loses its main source of estradiol. Estradiol is necessary for a man to feel right and funciton properly. It should be kept within a very specific range.
So if you use anabolic steroids but dont find a way to replace the estrogen, you leave yourself in a bad spot, to put it as simply as possible.
There are other ways to replace estrogen, but using testosterone is the most productive way as it also is excellent at building muscle.
Then theres also the issue with the different classes of steroids. Some steroids yield results by binding to and activating the androgen receptor. And some steroids appear to yield results by some other means. These are called class I and class II steroids. Testosterone is both class I and class II.
There is plenty in this post to serve as starting points for further research. Enjoy your reading.