If you want to score you need to change your aim all the time. If you don't and for example allways aim high on the goalies stickside -  the goalie's going to notice and expect it. Vary your aim and the goalie will guess wrong, making the wrong move from time to time, and thus increasing your chances to score.

Scoring is about forcing the goalie to leave an opening and then exploiting that opening.

In this clip I take two slapshots. First time, I aim low stickside and the goalie makes the save. Next time, taking a slapshot, the goalie guesses I'm going to aim low again and goes down with his pads, leaving the topshelf open.