Relationships are important for all people, and to build them, we have to share our emotions and be honest about our feelings with others. But not everyone is comfortable or even capable of opening up this way. A character who fears rejection or pain may be reluctant to “go there” emotionally. To avoid risking this vulnerability, they'll take on behaviors and habits that keep people at bay, resulting in loneliness and isolation from others.

Difficulty making eye contact when emotions are high
Sticking to surface-level conversations
Ending relationships before they become serious
Engaging in one-night stands to avoid having to connect
Not volunteering information about their past
Staying busy—always moving—to avoid connecting with others
Knowing their own emotional sensitivities and avoiding triggers for them
Having an exit strategy when they're out with others
Being highly independent and self-reliant
Appearing distant, self-absorbed, or uncaring to discourage others from wanting to get close to and confide in them
Adopting a look that makes them seem intimidating or unapproachable 
Wearing earbuds or headphones to avoid small talk in public spaces
Answering questions with as little actual information as possible
Deflecting conversation away from emotional topics
Juggling multiple romantic partners to avoid committing to one 
Telling people what they want to hear to stave off conflict or high emotions