Because in general, no one person does or even can see the whole picture.
I like the Buddhist parable of the blind men and the elephant. To quote Wikipedia(Blind men and an elephant - Wikipedia):
“It is a story of a group of blind men, who have never come across an elephant before and who learn and conceptualize what the elephant is like by touching it. Each blind man feels a different part of the elephant body, but only one part, such as the side or the tusk. They then describe the elephant based on their partial experience and their descriptions are in complete disagreement on what an elephant is. In some versions, they come to suspect that the other person is dishonest and they come to blows. The moral of the parable is that humans have a tendency to project their partial experiences as the whole truth, ignore other people's partial experiences, and one should consider that one may be partially right and may have partial information.”
I believe that different people are differently equipped(gifted) by God to comprehend and operate in different spheres of human experience(1 Cor 12–26). Some are born to work with people, encouraging and leading them and they understand psychology very well. Others were born to build things and they understand the physical world well. Others were born with insatiable curiosity and a good memory and are very knowledgeable about many things. In a community, we are each a part of a body. Some parts are more visible and some are hidden, but none is less valuable than another. All parts are necessary for the body to function. This is a beautiful design because it fosters mutual appreciation, respect and unity. It creates an opportunity to love others who are weaknesses in our areas of strength may make them less lovable to us, and motivation to love them because we also need them. It elevates relationship beyond mere personal enjoyment because we realize that we need each other to understand and do more than we can on our own. I believe this is especially true in marriage between a man and women, with their very different but complementary viewpoints and abilities.
This is why we should strive to understand differing viewpoints when we hear them and hold our tongues until we have. As the bible says, in James 1:19, “Know this, my beloved brothers: let every person be quick to hear, slow to speak, slow to anger” Even if the differing viewpoint is wrong, being confronted by it tends to refine and strengthen one’s own viewpoint as they defend it.
I hear that Ralph Waldo Emerson once said, “Every man I meet is in some way my superior, and in that, I can learn from him.” I think this is a great way to regard others.