Your body is built for survival, not to be balanced and aesthetically pleasing. When you're lifting weights you're stimulating the muscles to grow larger and stronger. This is an adaptive response to physical work.
However, just doing an exercise doesn't guarantee that you'll be stimulating growth in the muscle you want to maximize. When doing a bench press your body doesn't know that your goal is to build a big chest. All it knows is that a big-ass weight is trying to crush you and if you don't lift it you'll die.
So in the interest of your survival, your body will use the muscles better suited to get you out of this menacing situation. If your triceps or deltoids are overpowering your chest, then your chest might not get fully stimulated from the bench press since the nervous system will "shift" more of the workload onto the overpowering muscle(s). This is why some guys get a big chest simply by bench pressing while others build nothing but triceps or shoulders.
Just because you're doing what's traditionally known as a "chest exercise" doesn't mean that you'll be stimulating maximal pectoral growth. Sometimes you have to force your body to use the muscle you want to stimulate. This is what the "squeeze press" does.
The Squeeze Press
The squeeze press is a dumbbell bench press with one slight difference: the dumbbells are kept in contact with each other at all times and you're actively squeezing them inward (against each other) as hard as possible. This simple action will shift all the stress onto the pectorals.
It's very important to understand this point: you should squeeze in the dumbbells as hard as possible during every inch of every single rep. This is what makes this exercise effective.
You need to use hex dumbbells and keep the palms facing each other. This is easier on the wrists and works the sternal portion of the pecs to a greater extent. No hex 'bells? Throw a foam pad between two regular dumbbells.