Easy question...new to Whisky

Hello, I have recently learned of my love for scotch. I have a bottle of 12 yr. MaCallans currently. Here is my question: I store it in it's leather bag, in a kitchen cabinet (away from all light), in a room that gets no warmer at times than 72 degrees. But, it only tastes like new for a week or so before I notice a change. After a month or two it no longer resembles its original taste. Why does this happen? Any ideas / suggestions?