they are zoned out, unself-conscious, or concentrating on something. That is because they are not putting on the social facade people usually use when they are more self-aware, so they seem more genuine.
Also, they are not focused on me, so my typical anxiety over how weird I might look or what they expect me to do temporarily abates, and I can go back to being pre-occupied with my own thoughts instead of putting forth the effort to be "outside myself" to converse, so I can focus on them more and notice their attractive traits more easily.