Why is it that so many men leave public restroom toilets gross and unflushed for the next user?

I travel a lot for work and recently while on a road trip and pulled into a rest stop to use the restroom. I walked into the men's restroom and saw that the urinals were being use with a line for them, so I walked into one of the toilet stalls. Found it to be unflushed/gross and full of toilet paper. This is certainly not the first time I've seen this in public restrooms. It just disgusts me. Why do guys not flush and just walk and leave a mess for someone else? I don't understand how someone could do that. I am a very clean person and always flush. Now that I am asking this... do you guys flush? I just have to ask and wonder how some can be so dirty and careless in a public bathroom that others have need to use too? I don't know about the women's restrooms, but can only speak for men's restrooms from experience.