Unless you have a compelling reason to change fingers, it’s perfectly rational and musical to simply re-use the same finger.
In other words, changing fingers is not a rule. There are, however, at least two compelling reasons to consider changing fingers:
- Changing fingers may be required to get your arm, hand, and fingers into an easy position to play what comes next.
- Changing fingers, done right, may make things easier to play at very fast tempos, since you don’t have to keep “reloading” the same finger over and over again (down-up-down-up-down-up-etc.). Another finger can already be moving down while the finger that has just played is still releasing the note.