I didn't always have aphantasia, but I believe I ended up with it because when I was very young I had intrusive visual thoughts and developed a habit of blocking them out. By the time I was 30 I could barely remember being able to visualize things in my head.

I was 33 the first time so tried cannabis and I had vivid, colorful visualizations. It was intensely satisfying. I've since regained some ability to visualize in an un-altered state.