This is from Walt Whitman’s I Sing The Body Electric. Even introverts get this feeling sometimes–at least with the (few) people they really like:
I have perceiv’d that to be with those I like is enough,
To stop in company with the rest at evening is enough,
To be surrounded by beautiful, curious, breathing, laughing flesh is enough,
To pass among them or touch any one, or rest my arm ever so lightly round his or her neck for a moment, what is this then?
I do not ask any more delight, I swim in it as in a sea.