Herein, the five aggregates are impermanent. Why? Because they rise and fall and change, or because of their non-existence after having been. Rise and fall and change are the characteristic of impermanence; or mode alteration, in other words, non-existence after having been [is the characteristic of impermanence].4
7. Those same five aggregates are painful because of the words, “What is impermanent is painful” (S III 22). Why? Because of continuous oppression. The mode of being continuously oppressed is the characteristic of pain.
So as we know, whatever is impermanent must be stressful. The word "pain" seems to take it to a whole new level though. I'm having trouble with this because pleasures are impermanent and the thing is, when I experience pleasure, I don't feel oppressed at all. Are we therefore supposed to see pleasure as pain in the way that a vampire sees the sun as pain? Are we supposed to contemplate all feelings so that they eventually are all treated as pain during their occurrence?
Buddhaghosa says that the characteristic of impermanence is "rise and fall", or "non-existence after having been". Then in the next paragraph he states that what is impermanent is painful. So the meaning I get is: "The falling away or non-existence of what has been existing is painful". The problem with this is that it would mean that pleasant feelings are painful only on their passing away, and painful feelings are as well. To counter this, one can say that pleasant feelings are painful only on their passing away, but painful feelings are painful during their occurrence. But this doesn't make sense, because all feelings are painful, and it contradicts what he said.