The Rebbe teaches:
The Zohar explains the difference between Noach and his righteous successors: Noach “did not pray for the world,” for the members of his generation, and therefore the waters of the flood are called “the waters of Noach,” i.e., the flood is attributed to him.
In contrast, Moses displayed self-sacrifice by demanding that G–d forgive the Jewish people for the sin of worshipping the Golden Calf.
However, the Midrash tells us that Noach rebuked the people and encouraged them to repent, implying that he did care about the state of his generation. If so, why didn’t he pray for them and plead to G–d to have mercy upon them?
The explanation: Noach ’s rebuke did not stem from an altruistic desire to help the people, but solely from the desire to relieve himself of G–d’s command.
As Chassidus says, “Noach did not devote himself in his rebuke and warnings to inspire them to repent,” because the main purpose of his rebuke was to fulfill the command that he had been commanded. This explains why he didn’t act with Moses’ self-sacrifice by going to the trouble to pray for them.
 Zohar 1:106a.
 Shemos 32:32.
 Sefer HaMa’amarim 5705, p. 29.
In my own words, with some explanation:
It is written, “You shall surely rebuke your fellow Jew” (Vayikra 19:17). We are obligated to rebuke our fellow Jews (see Erchin 16b for a discussion of just how far this obligation extends) and encourage them to do the right thing. And yet this means far more than simply speaking words of rebuke to them. The main thing is that the rebuke stem from a genuine concern for their welfare.
When Hashem told Noach to rebuke the people, He meant to do so with true concern and persistence. But Noach only followed “the letter of the law.” He rebuked, but his rebuke stemmed only from a desire to obey Hashem, and not from a true concern for the people, and so it fell on deaf ears. The consequences of this lack of proper concern for his fellow humans were so far-reaching that Noach is considered partly responsible for the tragedy of the flood, and this is why its waters are called “the waters of Noach.”
The same is true concerning the task of spreading Yiddishkeit and Chassidus to our fellow Jews. Although activities to disseminate these teachings are obligatory (and especially for Chabad chassidim, who have been charged to do so countless times by the Rebbe), they will only bear fruit when they stem from a true love and concern for our fellow Jew and a desire to help him—and not merely the wish to “do our duty.” If we neglect to refine ourselves and develop our love for our fellow Jew (see further explanation of this here), and the result is that our efforts to teach others meet with failure, then we are partly responsible for the ongoing non-observance of those whom we were in a position to inspire.
Noach’s lack of true concern for others was demonstrated in his neglect to davven (pray) for them. Thus, perhaps one way of discerning whether our efforts to teach others stem from true love is whether we feel moved to take some time to davven for them, seeing that they are not yet privileged to know about the vast, rich treasures of Yiddishkeit and Chassidus that are the heritage of every single Jew.
Moreover, it is explained in the HaYom Yom of 3 Adar 1 that one should love every Jew, even one whom one has never met, and all the more so a member of one’s own community, i.e., those whom one knows well. Based on this principle, it would seem that we should davven in particular for those whom we know personally.