Why did you laugh?

I originally heard this joke in Spanish so here's my best shot at translating it:
4 men were walking in a forbidden forest when they found delicious fruit that belonged to a notorious group of savages. The savages found them eating their fruit and decided to punish them. The leader of the savages was a clever man so he smiled and told the men to pick out any fruit they wanted and that they could have it as long as they left the forest. t. Excited, the men picked out their favorite fruits ready to eat them.
Suddenly the leader said to each of them, "Now, if you can stick 100 of them all up your asshole without making a noise, we will let you go. If you don't, we will kill you."
The first man went and he had chosen raspberries but after the 15th, he cried out in pain and they killed him. The next man went and he chose strawberries and after the 25th one he cried out and was killed. The third man was up and he had chosen grapes. He had gotten up to 99th when he burst out in laughter.
Astounded, the leader asked "Why did you laugh? You were one grape away from living another day! Why??"
The man replied, "The other guy chose the watermelons!"