Does marriage mean I lose my Individuality since we will be one? Is it a sin if don't share everything on my mind with my wife? Does it mean no privacy for me?
The concept of being one is found in a number of relationships in the Scriptures. For example, "I do not ask on behalf of these alone, but for those also who believe in Me through their word; that they may all be one; even as You, Father, are in Me and I in You, that they also may be in Us, so that the world may believe that You sent Me. The glory which You have given Me I have given to them, that they may be one, just as We are one" (John 17:20-22). Jesus and the Father are one, but they remain the Father and the Son -- two entities. Jesus prayed that all Christians be one as well, but we each retain our own individuality.
Being one -- being unified -- means that we share a common purpose and common views. "Now I exhort you, brethren, by the name of our Lord Jesus Christ, that you all agree and that there be no divisions among you, but that you be made complete in the same mind and in the same judgment" (I Corinthians 1:10).
When you get married, your life no longer simply revolves around you. You and your wife will have formed a family. Your decisions will revolve around what is best for the family. It means when there are disagreements about the goals, you'll sit down with your wife and discuss the problems and come to a resolution from the viewpoint of what is best for the family.
You'll still have your friends and she will still have her friends. You will at times go off to do what you enjoy and she'll at times go off to do what she enjoys, but most of the time you'll be doing things together as a team.
This is why you are looking for a spouse who will be your best friend. You are going to be companions for the rest of your lives. Far too often people forget this and simply look for someone who excites them sexually. Such people bounce from person to person because excitement always dies down as you get to know someone. However, the companionship of marriage grows stronger as the years pass.
Your answer has changed my views. Thanks.