Using too much soap makes it harder to fully rinse away soap residue. And one of the reasons soap/detergent is so great for cleaning, is that it attracts dirt. This is great if all the dirt is rinsed away from the surface we were trying to clean. But if all the soap isn't gone...then the surface will continue to attract more dirt.
We see this a lot with carpet cleaning. If all of the cleaning residue isn't removed from carpets during cleaning, then it makes it much more likely that dirt will stick to the carpet surface, making it dingy prematurely. It also happens with washing machines and dishwashers. Too much detergent in either appliance can cause over-sudsing and potential damage to the appliance and surfaces around.
Follow the directions and you'll get better results. Extra isn't always better.