If one looks at the parsha one can see, in the commentary of Rashi, z”l, the elements of this picture. Look there and you will see that the snakes were sent upon Clal Yisraelbecause of their complaints against the ways of Moshe and of Hashem, Yisborach. These complaints were brought about because of the difficulty of their journeys at that time. What was difficult? That which was difficult was that they saw that they had turned back in the direction from which they had just come. They thought: here! we were just about to enter the land and now we have to turn back?!
Their complaints were unjustified and they were severely punished. From this we should learn the level of emunah that is expected from us: that even when it may appear as if we have stopped making progress in our lives and everything is turning around for the worse; and that we are becoming more distant from our goals; even here, we must have full confidence and security in Hashem‘s complete leadership of our lives, and understand that, in truth, the distancing is for the sake of coming closer. Hashem expects us to live with pure and simple emunah, and not let our happiness with and connection to Him be contingent upon the challenges that He sends before us.