One might think that making a vow or an oath is a positive thing, but the sages of the Mishna (Tractate Nedarim, chapter 1) teach us that these are not usually made by virtuous people, and therefore, a person who makes a vow ?as the vow of a virtuous person? does not really take on the prohibition since it is not the way of the virtuous to make vows.
Furthermore, the Talmud makes a determination in very strong language:
One who vows is considered as if he built (a personal) altar (outside the Temple, which is prohibited) and one who fulfills (this vow is considered) as if he sacrifices on offering on it.
(Babylonian Talmud, Tractate Nedarim, page 22)
We could have assumed that the Talmud is only referring to negative vows, like a person who vows to do something inappropriate? but Maimonides notes that the Talmud is actually referring to vows focused on positive actions. He writes as follows:
Although [taking vows] is an element of the service of G-d, a person should not take many vows involving prohibitions and should not habituate himself to taking them. Instead, he should abstain from those
things from which one should abstain without taking a vow. Our Sages stated: “Anyone who takes a vow is considered as having built a private altar.”
(Mishneh Torah LaRambam, Hilchot Nedarim, chapter 13)
We are talking, then, about ?those things from which one should abstain?, in the words of Maimonides, so why is that person compared to a man who transgresses the serious transgression of building an altar to sacrifice an offering?
What is an ?altar?? In ancient times, before King Solomon built the Temple in Jerusalem, anyone could build an ?altar? and sacrifice something on it. Jewish ritual was personal and individualistic. But when the Temple was built on the Temple Mount in Jerusalem, it was determined that it was no longer permissible to sacrifice offerings on private altars. The relationship between the Jewish nation and G-d was a national one, and no person, no matter what his status, had the right to separate himself from the public by making another place where sacrifices could be offered.
Now we can understand why the Talmud compares a person who makes a vow to one who builds an altar and sacrifices offerings on it. The person who builds an altar is trying to be different from everyone else. The entire Jewish people brings sacrifices to the Temple, but he builds an altar for sacrifices in his home. Similarly, someone who makes a vow wants to find his own way to worship G-d, by creating new prohibitions unique only to him that express the specific spirituality he wants.
The Torah objects to this. Withdrawal from society is not desirable. Ever. No one argues that a person can and should find his own way to express his uniqueness and find the path that suits him best ? but only within the framework of general, national Judaism. There is no need to build an altar or make vows to express a specific and special personality. One can fulfill the commandments that all Jews fulfill and find within them one?s own unique hue and the manner that suits each and every individual.