Imagine Chi, or energy, as water flowing into your space. You will see that water will rush very fast through the two or more directly aligned doors without actually slowing down and nourishing the space. It is especially important to avoid a direct alignment of the front door with the back door. The reason you want to avoid this situation is because all good feng shui energy coming through the main door will easily escape through the back door without having a chance to circulate and nourish your home.