I think he means that it is irrational to ponder death when those moments can be spent living life productively. Not sure if I agree -- doesn't the thought of one's death often propel us to great action, while lack of such thoughts leads to complacency? Anyways here is the the proof from the Ethics:

Proof.— (67:1) A free man is one who lives under the guidance of reason, who is not led by fear (IV:lxiii.), but who directly desires that which is good (IV:lxiii.Coroll.), in other words (IV:xxiv.), who strives to act, to live, and to preserve his being on the basis of seeking his own true advantage; wherefore such an one thinks of nothing less than of death, but his wisdom is a meditation of life.