What is it with the "You had to be there" experience?

[d's girl]

I don't understand how something can be riotously funny when you're present for whatever the experience is, and yet it doesn't translate in the retelling. What is this "you had to be there" phenomena? How does it happen?

I guess you had to be there. 

[d's girl]

lol!

[Aggie]

It happens often in Texas.

[TeeBubbaDee]