By Rabbi Yirmiyohu Kaganoff (RabbiKaganoff.com)
When our ancestors accepted responsibility to observe the Torah, they did so by performing bris milah, immersing themselves in a mikveh, and offering korbanos. A non-Jew who chooses to join the Jewish people is entering the same covenant and must follow a similar procedure (Kerisus 9a).
The privilege of becoming a ger tzedek requires one to follow very exacting guidelines. On a technical level, the ger is accepting responsibility to perform mitzvos. Through the gerus procedure, he creates an obligation upon himself to observe mitzvos (Birchas Shmuel, Kiddushin #15).