Rashi explains that Moses was following the example of Jacob, who only rebuked his sons when on his deathbed. The reason he had not rebuked Reuben, for example, earlier, was that he had feared that if he had done so, it would cause a rift between father and son, to the extent that Reuben would leave him and join his brother Esau, who stood for exactly the opposite of what Jacob stood for. Moses was presumably afraid that, with most Israelites probably on a lower moral level than Reuben, and Moses being less eloquent than Jacob, any attempt by him to reprimand the Israelites before this moment would be equally doomed to failure. So he delayed forty years before carrying out the mitzva of "hocheach tochiach"! This indicates how delicate the performance of this mitzva is, and how careful we should be with it.
The Gemara (Erchin 16) quotes R' Tarfon as saying: In our generation, there is no-one capable of giving a rebuke. R' Elazar ben Azaria argues: No, the problem is that there is no-one capable of receiving it. R' Yochanan ben Nuri argues: You are both right: no-one nowadays is capable of either giving or receiving a rebuke! What does this say for our own, orphaned, generation? We should be extremely careful about the performance of this mitzva. The Gemara (Yebamos 65) says that if you have reason to believe your rebuke will not have the desired effect, then you must not make it! The point here is that you must not think: "Well, now it's up to him to accept my rebuke, or not; if he doesn't, he doesn't; in any case I've done my duty by admonishing him." The fact is,you may have done a whole lot of damage by admonishing him! You may have embarrassed him -- a serious sin by itself -- and you may have driven him away from Judaism.
With Tisha Be'Av aproaching, we must think about the reasons for the destruction of the Temple. The Gemara gives many reasons for this, but they all seem to have a common theme: a lack of concern for other people, particularly for their feelings, even in the process of performing some mitzva. We should all take this lesson to heart.The late Rav Kook said: There is only one sure cure for the sin of "sinas chinam" (causeless hate), and that is "ahavas chinam" (causeless love).