Pineapples have these little microfibers in them that can be a bit rough on the tongue if you eat much. It just happens.

I can usually get about halfway through a pineapple before it does that.

Apparently, soaking the pineapple in sugar water for a while before eating it helps prevent that, probably by leaching out some of the acidity, which I'm sure contributes to the wear and tear on the mouth.

I never bother though, I just eat it until it hurts. XD