They can become touchy and defensive when certain people try to ask about their lives, like a lonely teenager may feel grilled and interrogated if his parents good-naturedly ask him about his friends, when he doesn't have any.
They may dread situations where their feared subjects are more likely to arise, like when their friends are all sitting around and drinking.
If you have a secret no one may think it's a big deal if they found out what it was, but they'll form a poor impression of you if you're always closed-off and cagey.
Their imagination may run wild and they'll assume something worse about you than what you're actually hiding.
There are a number of reasons why a person can become emotionally unavailable, and many different levels of emotional unavailability exist.
In some cases, partners can become emotionally unavailable even in long term committed relationships.