For the Lord spoke to Moses, saying, "Only the tribe of Levi you shall not list, and you shall not take a census of them among the people of Israel." (Bemidbar / Numbers 1:48, 49; ESV)
The Hebrew name for the fourth book of Moses is Bemidbar, meaning "in the wilderness" which is referenced in the book's first verse. The common English name, "Numbers," is most likely due to the numbering of the people with which the book begins. The Hebrew name is more fitting, however, since most of the book is about various incidents that Israel experienced during their years in the wilderness prior to entering the Promised Land.
Readers tend to find the beginning of this book difficult, since we don't easily derive personal benefit from census data. Yet these are the types of passages that remind us that the Bible is more than a moral and religious document, it's a historical document wherein the true God reveals himself through the real lives of actual people.
The census with which Bemidbar begins is not a general one such is common in much of the world today. Rather, it was a counting of the men who were of military age and ability, a very important practical exercise to gauge the military resources of the nation. Every man from the age of twenty and up who was "able to go to war" was to be included. We are not told what conditions excluded someone from military service, but we can assume that it would have been obvious to them. A maximum age did not seem to be one of those conditions.
All the men twenty and over were to be counted except those of the tribe of Levi. The Levites were not eligible for military service, for they were set aside by God for service to him.
There is much that can be said about the special role that the tribe of Levi played in the life of the nation of Israel. But however we look at it, they were not like the rest of the nation. While some people like being different, it seems to me that most of us want to fit in with the crowd. Remember not only were the Levites not counted along with their kinsmen, when the time came they would be the only tribe not fighting.
Perhaps some of the Levites were glad to be excluded from military service, but others probably had a hard time staying away from the battle. Some among the fighting men may have resented the Levitical military exemption, while others may have wished they were Levites themselves, so that they would not have to face the dangers of war.
But how Levites and non-Levites felt about this was irrelevant. One's role among his people was determined by God alone as the exclusion was based on birth. There was nothing one could do about it, except to accept it and live accordingly.
Even though the Levitical system is not functioning today, God continues to set aside people for different roles based on his own determination and for his own purposes. Sometimes God's determination is clear as in the distinctions between men and women or parents and children. Other times it is not so clear as God leads different people to take on different roles. But whatever the reasons for our differences, the sooner we discern what they are, understand their particular purpose, and fulfill that purpose, the more effective we will be in life.