1. The attribute of modesty:
Being modest in all one’s ways:[1] The attribute of modesty is praised by verses of scripture in various places. The sages commanded that every person be modest in all of his ways, and not act in an immoral manner, even when there is no one else around and one is in private.[2]
Being modest even at night and in private:[3] Even when one is alone at night in the privacy of his room he is required to act with modesty and bashfulness before G-d, whose glory fills the entire earth, and to which darkness is like light. [Thus he should not say I am in private, who will see me?[4]]
The reason behind the command to be constantly modest:[5] One is obligated to constantly have the fear of G-d upon him. Therefore he is to be modest in all his ways, as acting with modesty and bashfulness brings one to humble himself before G-d.
[1] Basra 2:1
[2] The Alter Rebbe here motions the reader to Hilchos Tznius. This refers to chapter 240. Vetzaruch Iyun as to which law there he is referring to.
[3] Admur Basra 2:1; Kama 2:1; Siddur “Seder Netilah”
[4] Kama ibid
[5] Admur Kama 2:1