The beginning of your story carries more weight than any other part of your work. This is simply because it is the beginning. The reason for its prominence is similar to seeing anything for the first time. Your senses are attuned. It’s analogous to seeing a person for the first time. When someone walks into a room, a person you’ve never seen before in your life, you experience that person intensely.  The next time that person enters the room, though—i.e., your second line or second paragraph—everything is calmer, the intensity is far less. You may have certain new impressions or observations, and the first ones may be refined, but that second look is nowhere as potent, as intense, as the first. As it is true in life, it is true in writing.