I don't know what makes a guy top, bottom or versatile but here are a few of my thoughts.

I've only topped except for one or two times I tried bottoming with my bf which didn't last long as I found it uncomfortable. I had been feeling really horny and wanted to give it a go, but as soon as he was inside it just made me lose all my hornyness and I didn't even want to fuck him afterwards.

It seems like a much bigger step to bottom than to top from masturbation or straight sex. Also I pretty much like to be in control when it comes to sex, I know you can be in some positions, but normally it's more of a top than a bottom thing. Lastly, it's just more of a mental turn on for me for some reason.