Think of changing teams in the NBA like moving from one house to another. You might go about your daily business in similar fashion, but so many of the little things are different.
You have to take a different route to work, you have a different floor plan to navigate at home, the grocery store is in a different location and you have new people to interact with. Even though your goals might not change, the little parts of your everyday life are certainly new.
Such is the case for NBA players when they put on a new uniform. The goal is still to score points and keep the opponents from lighting up the scoreboard, but the methods of achieving that goal are quite different.
New players surround you, new plays must be learned and so on. Oh, and you have to change houses as well.
It should come as no surprise that some players struggle initially when making the transition. For every James Harden, Kevin Martin or Kyle Lowry, there's a Dwight Howard, a star who is still learning how to fit with his new team.