On one occasion Ven. Sariputta was staying in Magadha in Nalaka Village. Then Jambukhadika the wanderer went to Ven. Sariputta and, on arrival, exchanged courteous greetings with him. After this exchange of friendly greetings & courtesies, he sat to one side. As he was sitting there he said to Ven. Sariputta: "'Stress, stress,' it is said, my friend Sariputta. Which type of stress [are they referring to]?"
"There are these three forms of stressfulness, my friend: the stressfulness of pain, the stressfulness of fabrication, the stressfulness of change. These are the three forms of stressfulness."
( http://www.accesstoinsight.org/tipitaka ... .than.html )
'Stressfulness of pain' and 'Stressfulness of change' are both fairly straight-forward in terms of why they should be considered forms of suffering. However, why do Saṅkhāras cause suffering? Also, since we are always in the process of creating or experiencing some conditioned thing, is there a persistent suffering that always accompanies our experience?