I believe everyone has experienced being in a room full of people and not being in the mood to be sociable (probably should have stayed home) or for some reason don't feel like you see anyone you (want) to attempt to connect with. You may feel intimidated.
Essentially you feel you don't "fit in" or you don't like the vibe.
In other instances sometimes people are naturally "introverted" and uncomfortable striking up conversations with strangers.
They'd rather have someone approach them.
Last but not least some people are arrogant or feel they're "better" than the people assembled together for one reason or another.
"Why am I'm wasting my time with this bunch of losers." mentality.
How we feel about ourselves, the mood we're in, and the circumstances taking place in our life determine whether or not we feel like socializing. If someone is depressed they most likely don't want to engage in meaningless conversations.
They'd rather enjoy their drink and people watch or simply leave after making a cameo appearance.