Why can only believers look forward to the Lord's coming?
Believers possess the firstfruits of the Spirit, so they can eagerly await the Lord's return and the lifting of creation's curse. Unbelievers lack that readiness; they shrink from His coming, wish it away, and are not prepared for Christ to intervene and set things straight.
Les explains in Romans 8 that the whole creation groans and travails in pain, waiting for the day when the curse will be lifted. He points out that Paul is looking only at believers, those who have the firstfruits of the Spirit. Because they have the Spirit, believers are the only ones who can anxiously look forward to the Lord's coming. The unbeliever, by contrast, shrinks from it, is not ready for it, and would like to think it all away, but he will not succeed because Christ is coming. Les also observes that the whole world is moving faster and faster toward that final moment when Christ will intervene once again in human history and set things straight.
And not only they, but ourselves also, which have the firstfruits of the Spirit, even we ourselves groan within ourselves, waiting for the adoption, to wit, the redemption of our body.