Focusing on yourself, your own needs, or other aspects your personal process.
Independence, or a desire for more or less independence.
Not wanting to participate in society, or wanting to act against it (perhaps due to beliefs or issues with authority).
Needing a break from the day-to-day activity around you.
Someone who purposely chooses isolation (such as a hermit or a loner) can represent your perception of that person (or whomever they represent) in one of the ways previously mentioned.
See also: Leaving a Person; Leaving a Place; Rejection; Divider; Abandonment; Left, Being; Stranded; Distance; Breaking Up