Depression happens most often, lasts longer and is more disabling, triggering higher suicide rates. That's a good question. One of the first antidepressants ever discovered was designed for treating tuberculosis. The consensus is that omega 3 is incorporated into cell membranes, making them more fluid. Every cell membrane has a layer of fat around it.
Receptors stick out of the cell.
They go from the inside of the cell all the way to the outside of the cell. They float in that membrane of fat. If the cell covers its privates with fat from double-cheese Whoppers, extra fries and chocolate milkshakes, it's going to make the receptors inflexible. Imagine how your TV reception would be if your satellite dish couldn't move to capture the signal. If you don't have a good diet, your membranes are going to be physically stiff, which changes receptor dynamics, changes ion channels and what calcium does in the cell.