So I was wondering. What caused some civilizations to associate a funeral with a somber gathering while others treat it as a celebration of a past life? Especially with the widespread concept of an afterlife I'm wondering if there were shifts in cultural opinions and what caused them.
As always, Herodotus comes to the rescue. And also as always, he needs to be taken with a grain of salt. From the 5th book of his Histories:
The Trausi in all else resemble the other Thracians, but have customs at births and deaths which I will now describe. When a child is born all its kindred sit round about it in a circle and weep for the woes it will have to undergo now that it is come into the world, making mention of every ill that falls to the lot of humankind; when, on the other hand, a man has died, they bury him with laughter and rejoicings, and say that now he is free from a host of sufferings, and enjoys the completest happiness.
You might be more successful in getting an appropriate answer in /r/AskAnthropology.