Apathy - a lack of feeling; a state of emotional indifference
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Laughter. Smiles. Shouting. Jumping. Roughhousing.
These are all going on around you. When someone punches your shoulder, you barely react beyond turning around and staring.
"Hey, what's wrong? Why aren't you joining in?"
You shrug and turn back the other way. Everyone around you continues to fool around and you can't quite bring yourself to care about the way, that by now, no one could care about you.