It might sound counterintuitive, but using too much detergent will actually leave your laundry dirty. This happens because the base components of detergent work in specific concentrations. When those concentrations are too high bad laundry mishaps can occur. So let's get into it, shall we?
The squeaky, dirty truth
The exact makeup of detergent varies from brand to brand, but the basic molecular structure remains the same. Soap molecules need to be amphiphilic, meaning they have both a "dirt-loving" and a "water-loving" end. The dirt-loving end, which also repels water, is typically made of lipids—the building blocks of fats. This is why detergents and soaps feel greasy. This end anchors itself onto stains.
When your washing machine spins and rinses, the water-loving end wants to go with the flow, yanking bits of the stain along with it. Using soap effectively requires water and mechanical action. Think of soap as a shovel and your arms as water. If you don't move your arms the shovel serves no purpose.
When soap clings to your clothes, the easiest way to get it out is by running water through it. However, washing machines are designed on the assumption that you're going to use the recommended amount of detergent. If you don't have enough water or spin, your clothes are going to come out visibly caked in soap or feeling greasy.
This happens a lot with the Delicates cycle. People use the same amount detergent as the Normal cycle, but the Delicates cycle doesn't spin as much so the soap molecules aren't pried away, leaving clothes feeling greasy.
Dye stains, dye!
Have you ever wondered why every brand of laundry detergent is blue? It's because they contain blue dye. The dye is used to counteract yellowing. Unsurprisingly, if you use too much dye it'll turn your clothes blue. This became super evident when Tide first introduced their laundry pods. When consumers used too many of the pods, it tinted their laundry.
Foaming at the drum
The fact of the mater is, the more detergent you use the more suds there will be. Newer washing machines like the Samsung WF569H100AG have sensors that check for any remaining dirt in the suds. Smart washers have programing that makes the assumption that you used the correct amount of detergent. If you use too much detergent, then you'll get too many suds. These extra suds won't pick up any dirt and will obscure the fact there are more stains to be removed. When the sensors see only clean suds then the machine will think that the wash is done, prematurely ending the cycle.
Finally, using too much detergent can damage your washer. Detergent that doesn't get washed away dries up as residue inside your machine. Repeated overuse of detergent causes residue to buildup, which eventually leads to blockages. In turn, these blockages force water to back up into places where it shouldn't be, like the control panel or your floor. More importantly, when your washer breaks down you can't wash your clothes effectively, so they stay dirty.
So how much detergent should I use?
Good advice can be found in Jolie Kerr's book My Boyfriend Barfed In My Handbag: "A good rule of thumb, if you're using the cap for measuring, is to fill it no more than a third... of the way up." The passage goes on to talk about how half a cap should be reserved for heavily soiled items. Of course, as ingredients for detergents change, you should always follow the instructions on the bottle.
We should also state, the best way to use less detergent and get cleaner laundry is to pretreat stains. Cold water is the quickest pretreating tool. Water keeps stains from settling into fibers, and the cold temperature prevents substances like protein from "cooking" onto your shirts. There's also a variety of stain removers and pretreatment sprays you can buy to limit the volume of detergent you'll need for each load.
That's pretty much all there is to say about the subject. The most important takeaway should be to use less detergent, and pretreat more.