Cicero develops an argument around this claim in Book 2 of On Ends, we'll need to find that section. The basic argument he makes is something like this;
- Epicurus holds that pain/fear/mental disturbance is an evil.
- It is not possible to be confident that one will avoid this evil in perpetuity.
- If the Epicurean cannot be confident of this, he cannot guarantee his continuing happiness, and as a result present happiness will be impossible.
- The Epicurean can never be happy.