Why are pleasant feelings "stressful" & "unpleasant"?

I read the following in the internet: > All feelings are categorically classed as unpleasant in the > Dhamma...'Three feelings have been spoken of by the Blessed One: a > feeling of pleasure, a feeling of pain & a feeling of neither pleasure > nor pain.,,,Now in what connection was this stated by the Blessed One: > "Whatever is felt comes under stress (yaṃ kiñci vedayitaṃ taṃ dukkhasmin)" > Why are pleasant feelings "stressful" & "unpleasant"? If all feelings are unpleasant, why are some feelings called "pleasant"?