It's just one of those things that either tickles you or it doesn't, I think. I'll try to explain why I found it funny, but if you didn't enjoy it the first time, you probably won't enjoy it even after hearing my explanation.

So it's pretty obvious from the start that this is a joke song. The singers' delivery is a little too heart-felt; the lyrics a little too cliché. So you know something funny is going to happen. Usually joke songs hit you with the punchline in the chorus, so you begin to wonder what the lyrics are leading up to -- gay cowboys is the obvious joke. Then they hit you with it: SCREAMING SKY GRANDDAD. It's such a different style from the rest of the song and video, so different from the "funny lyrics" twist I was expecting, that I couldn't help but laugh.

And he just keeps going. The funniest moment for me is at 2:28, when he shoots up by a few notes. He's impressively good at scream-singing, but he's old enough that his voice has lost the smooth quality that the younger singers have, so hearing him just nail this rusty old-man high-pitch scream that has no reason to be in any goddamn song had me literally crying with laugher.