Two things that I noticed among good BJJ practicioners around me that develop fast. First, they are looking for, admit, and work on their weaknesses. Stefer, even after dominating me, often asked me on what should be improved on his game. Secondly, they are not afraid to lose.
What are the similarities between these two traits? That's right. Both of them show that they are checking their egos by the door.
The question is, how crucial is this attitude toward one's progress on one's game? The answer is paramount.
Working on your weakness meaning that you are prepared to lose in order to work on your weakness - a step back to sprint forward. These fellas have the courage to get out of their comfort zone of using your most effective techniques and strategy, as opposed to settling down on winning against weaker opponents.
In fact, working on your weakness creates more progress to your overall game. Imagine someone with a lousy takedown. Anyone who fought this guy would've guessed that he will pull guard to take the opponent down, making his game very predictable. Now, by simply working on 2 takedowns will make his game significantly by tripling his option, thus making his game less predictable. On the other hand polishing on strong points may have less advantages over the same amount of period - people will know you'll after those positions and will try not to get to your setups, thus eliminating element of surprises, as well as lesser increment to your overall game.
So, check your ego by the door during training. forget about winning the sparring sessions, as the objective is "training" not "winning". You don't want to win small skirmishes - you want to win where it counts - on the streets and on the tournaments.