Bhagavad Gita

ಯದ್ಯದಾಚರತಿ ಶ್ರೇಷ್ಠಸ್ತತ್ತದೇವೇತರೋ ಜನಃ ।
ಸ ಯತ್ ಪ್ರಮಾಣಂ ಕುರುತೇ ಲೋಕಸ್ತದನುವರ್ತತೇ॥೨೧॥
yadyadācarati śreṣṭhastattadevetaro janaḥ |
sa yat pramāṇaṃ kurute lokastadanuvartate||21||

Gist of the sloka:
Whatever the leader does, the followers follow him. Whatever he uses as a reference, the world uses the same.
One who has been recognized as a senior in the society, one who is looked at as a knowledgeable person in scriptures, one who can provide guidance to the society, such a person is followed by rest of the society blindly. Not everyone has time, patience or skill to understand the scriptures and adhere to it. Due to which, the rest of the population, mirrors the actions of the leader with a blind assumption he would know what he is doing. This means more responsibility on the shoulders of the leader to be responsible for all; through his behavior and practices.
Arjuna is a recognized leader in the society and there is a need for him to be model for rest. We see this happening every day in our lives. We see great institutions being reduced to dust due to wrong leadership. Loss of - moral high or lack of dedication to duty and sincerity being the primary cause.
It is the human weakness that they get attracted and follow the bad examples much more due to its attractiveness [quicker happiness] than good examples [ as it requires mental stability, determination, persistence etc.,]. If a very good teacher teaches the subject well, but also smokes, the children remember the example of smoking much more than the subject. They will then imitate him. Therefore, the responsibility and ability to persist in the right path is much more important for the leader.