Paul wrote to the church in Thessalonica in 1 Thessalonians 1:3 "We continually remember before our God and Father your work produced by faith, your labor prompted by love, and your endurance inspired by HOPE in our Lord Jesus Christ."
Romans 5:3-5 "We rejoice in our suffering, because we know that suffering produces perseverance; perseverance, character; and character, HOPE. And HOPE does not disappoint us because God has poured out his love into our hearts by the Holy Spirit whom he has given us."
We are not talking about being optimistic. It is the blessed assurance of our future destiny and is base on God's love which is revealed to us by the Holy Spirit.