The Start of Avodas Hashem:

On the topic of the connection between hair and dinim, the Noam Elimelech zy"a (Hosafos Likutei Shoshana) writes: “One should be careful not to grow his hair long because the evil forces can latch onto hair, and one doesn't want to give them a place to grab onto. This means that cutting the hair is the start of avodas Hashem."