She is screaming because she is really scared.
Something is chasing her, so she is screaming for help.
He's shouting because he is angry.
When he's really angry, he often shouts like this.
She's sleeping because she's tired.
Last night she didn’t get enough sleep, so today she has no energy.
She’s crying because she’s sad.
She got some bad news a few minutes ago.
She’s smiling because she’s happy.
Her boyfriend just called and he is returning from a long trip.