In 6:2 the Torah moves on from discussing the concept of Sotah and moves to the topic of the nazirite. Rashi, commenting on the proximity between the sections regarding a Sotah and a nazarite, explains that a person who saw a Sotah in her disgrace will be shocked and become a nazir to separate himself from the worldly pleasures of wine..
There is nothing wrong with partaking in the pleasures of olam ha'Zeh, this world. As a matter of fact, it is explained that the reason why a nazir must bring a sin offering upon completion of his nezirut is because he prohibited from himself the pleasure of worldly items (wine) that Hashem permitted. In a sense it is a type of sin for him to have done so.
However, when a person sees the shock of a Sotah in her disgrace, her shirt being torn, her hair being cut, her korban, and in her disgrace her womb collapsing and legs failing, it will cause him to think about what happened. A person cannot live as a pedestrian/bystander. We are affected by what happens around us. This person saw the depravity of the Sotah and reacted by restricitng himself from certain pleasures that commonly cause immorality.
A person has to react to what happens. Seeing a Sotah is a shock to the system and deserves a reaction. Seeing anything deserves some sort of reaction. One cannot be passive.