Before Yaakov Avinu passed away, he called all his sons together and blessed each and every one of them. Reuvein, the eldest and the rightful heir to the privileges of the firstborn, was the first to receive his father's blessing. "Reuvein, you are my firstborn, my might and my first strength; destined to be greater [than your brothers] in kehunah and kingship. Hasty like water; you will not be foremost, since you ascended your father's bed" (Bereishis 49, 4). Rashi explains that in his hastiness he acted out of anger (when he removed Yaakov's bed from Bilha's tent) and as a result he lost his rights to kehunah and malchus.
Rav Wolbe (Shiurei Chumash) elaborates that one who acts out of haste cannot possibly be a king. A king must have the ability to remain composed in all situations, lest he act impetuously in response to sudden and disturbing news and possibly bring destruction upon the citizens for whom he is responsible.
The same holds true for a kohein. One cannot work in the Beis Hamikdosh if he acts rashly. The Gemara (Pesachim 65a) tell us that Chazal did not institute Rabbinic restrictions in the Beis Hamikdosh since the Kohanim acted with zerizus (alacrity) and therefore, there was no need for extra safeguards. What does this mean? Isn't someone who acts with zerizus more prone to making mistakes? Rav Wolbe quotes his father-in-law, Rav Avrahom Grodzenski, who explains that zerizus is not found in the feet; rather, it is found in one's head. Zerizus is not haste - which makes waste, it is a zeal and fastidiousness which ensures that one does not deviate an iota from how he is expectedto act. Hence, Reuvein who exhibited the trait of haste was not a candidate for either of the above positions.
Rav Wolbe continues, that at first glance it's hard to understand how this rebuke can be called a blessing. However, in truth, it was one of the greatest blessings possible. Yaakov revealed to Reuvein his underlying character trait. Such a piece of information is worth much more than its weight in gold! It has the ability to set one on the proper path for his entire life by making him cognizant of the middah which, should it go unchecked, has the ability to destroy his avodas Hashem.
Rav Yeruchom Levovitz would say just as an underlying negative middah has the ability to derail one from his avodas Hashem, similarly, each person has an underlying middah that through it he has the ability to rectify all his faulty middos. We might need someone else to identify it for us, but once we do find out which middah resonates strongly throughout our character we should cherish the piece of information like we would a precious gem. It is the key to success in our avodas Hashem.