Vijay Gill shared his belief that ISPs go through a Peering Policy lifecycle as shown in the figure below.
When the networks first enter the peering ecosystem, they tend to be more open and willing to peer with anyone that can help decrease their transit expenses.
During this time, Public Peering and Open Peering is preferred as a means to aggregate large numbers of small peering sessions.
As the network grows the cost of maintaining so many peering sessions compared to the value derived leads the ISPs to prefer to peer with fewer larger peers. Once these upstreams are peered, there is less value to peering with the downstreams, so a Selective Peering Policy may be adopted and de-peerings may occur. When an ISP becomes a Tier 1 ISP they have access to all routes in the Internet Region and they will adopt a Restrictive Peering Policy.