Because I'm happy
Clap along if you feel like a room without a roof
A roof, which can be thought about as a “ceiling”, is the highest point in the house, the limit. “Without a roof” symbolizes that there is no barrier, and that is to show how limitless his happiness is.
On the flip side, a “room without a roof” will receive a lot of rain, and can be very gloomy. Pharrell doesn’t just want the happy to join him; he wants the sad to join him too.
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