When pouring a fizzy drink why tilt the glass?

Ok stupid question but here goes – when pouring a fizzy drink (beer, coke, whatever) does it help to tilt the glass sideways.

If you pour the drink with glass standing up the bubbles are much more intense, where as if you tilt the glass the bubbles are much less.

Why is this? I thought it was surface area of the liquid being greater but I think this is rubbish… any thoughts?