After married, the world becomes different??

What is the meaning of marriage? Why do people need to get married? Is it all because of love? I am confused when I see my sister's preparation of wedding. All of us in my family was under in stress for helping her wedding. The bride can shout loudly and says out loudly that she is very stressful and can do anything she wants. Why she cannot help her family to reduce stress? She is too selfish to only see her own side and cannot consider us. Why I have a sister like this, I really did not understand her well. I did not know how to communicate with this kind of person. She never thinks that the world is so beautiful and the family is so loving her. She always think the bad side of the world. God, please help us , help our family , help my sister, we are so vulunrable to treat our emotion, we are so stupid as don't know how to love the others especially our closest family members. Please Please teach me, I really want to learn all of this.