Originally Posted by Paul Markham

When a person orgasms the brain releases chemicals which produce a feeling of elation and well being.

Seeing the orgasm or even better "Cumming together" with the actor produces a surge or reminder of this feeling.

Women are different for them the orgasm is the same feeling, but cum on her face and she knows it's a trip to the bathroom when she wants to be held in a loving embrace.

There is also the feeling of power we get from the orgasm, seeing it also reminds us of that feeling.