swa66 - 4:10 pm on Jan 10, 2013 (gmt 0)

What most out here probably are unaware of is that there's a number of different relations between ISPs as well.
It's not an easy matter to fully grasp. I'll give it a try:

Essentially traffic nowadays is mostly asymmetric in bandwidth use (consumers pull more in than they send out and publishers send out more than they pull in).

Whenever ISPs are discussing peering (i.e. build connections between 2 ISPs) they look at traffic level between themselves (and their respective downstreams) and if it's not a "I send about as much to you as you send to me" then it'll not be a peering (peering is between equals), but the dominant one will try to charge the other.

So who's dominant ? Well that's a question of who's gotten the bigger tool you might say, but what always plays in it is who's sending more traffic to the other is the dominant one (i.e. the one having more publishers is more dominant).
Now location, network, status, reputation etc. all count into it as well - but traffic levels in each direction are by far not unimportant.

So for an ISP it is financially interesting to not be a downstream of others - it makes them more profitable by not having themselves to pay other ISPs for their own internet connection.

Players like Google are really big and in this game. So connecting them to other ISPs means they always would end up as the top dog - essentially generating them income for there's no internet if you cannot reach Google.

Little local players offering things to consumers always end up as the downstream of other bigger players.

If you ever wonder why a datacenter offers more bandwidth for cheaper than what you can manage at home while they'd have to flood the bill for power, cooling, physical security etc. as well ... yep: the datacenter if big enough will not pay as much for their big pipes to the rest of the Internet than the ISP that would connect you and all your neighbors at home would.

If you're interested in more: read up on "transit-free": that's the top status - ISP that doesn't pay any other ISP for traffic (it's in reality a small gentlemen's club and becoming a member is very hard).