It is stated in Wikipedia that


  an IPv6 header does not include a checksum.


What are the reasons that were behind this decision?

One of the ideas around IPv6 was to speed up packet forwarding. To that end, several decisions were made. For example, the IPv6 header was greatly simplified and is a fixed length, unlike the variable length IPv4 header. Also, you cannot fragment IPv6 packets along the path, the way you can for IPv4, because packet fragmentation is resource intensive.

Not having a checksum in the IPv6 header means that an IPv6 router does not need to recalculate the checksum to see if the packet header is corrupt, and recalculate the checksum after decrementing the hop count. That saves processing time and speeds up the packet forwarding. The logic is that the layer-2 and layer-4 protocols each already have a checksum. The layer-2 checksum covers the entire IPv6 packet, and the layer-4 checksum covers the transport datagram.

Where UDP has an optional checksum for IPv4, it is required for IPv6.
