Why are pleasant feelings "stressful" & "unpleasant"?
This is what it says in MN 44 sutta.

&quot;Pleasant feeling is pleasant in remaining, &amp; painful in changing,
friend Visakha. Painful feeling is painful in remaining &amp; pleasant in
changing. Neither-pleasant-nor-painful feeling is pleasant in
occurring together with knowledge, and painful in occurring without
knowledge.&quot;