Well consider the main door of your house as your face and ask yourself this questions:
- Why should my face look clean?
- Why my facial expressions say a lot about me?
- Why men/women care a lot about their face in comparison to the other parts of the body?
- Why everyone likes to see a happy face compared to a sad face?
- Why do we all love calm and smiling babies not the crying ones?
Quoting from this webpage,
Similarly, main entrance door holds an important place in your house. It creates the first impression of your house. It is not just a source of the entrance of people, but energy as well. Thus, the main entrance has been given prime importance in Vaastu Shastra. It lets the flow of cosmic energy in or outside the house. Cosmic energy is that promotes your health, wealth and harmony.
Everything and anything around you affect you positively or negatively if not placed correctly. Similar is the case with the main entrance door. It is the entry and exit of the positive and negative energies. Vaastu for main entrance door should be taken seriously, because of course you don’t want the negative energy to get stuck in your home or at your workplace.
Nevertheless, the main door of your house is where the most of the energy enters and therefore this door determines the flow of vibrations experienced by the household and its members.