<human>: Can you condense the text into a few key points?

1So my dog bit someone today. He's never shown aggression to people before.

A member of the family who doesn't live here had the dog on the couch with him and with no warning snapped at his face and drawn blood. This member of the family is here pretty much every day and takes him for walks and stuff, has done since we got him 10 years ago.

I'm trying to work out why he's done it, but it's racking my brains. He knew he had done wrong as soon as he did it, but then seemed very nervous around the person he bit. Now he's gone the dog seems just as his normal happy self and I can't put my finger on the reason for the sudden outburst. 

He is acting his completely normal happy self around all of us who live with him. Have him sleeping at the bottom of my bed at the moment just because the other 2 people who live here are women who wouldn't really be capable of doing anything if he decided to try and eat anyone again.
<bot>: Dog snapped at member of the family who is a large part of his life. No idea why. Any ideas out there as to why he might of done it?