Fear, when not controlled, gives evidence that a person does not believe that God is telling the truth and that He cannot be trusted to have one's best interests at heart. This rejects Jeremiah 29:11: "For I know the thoughts that I think toward you, says the LORD, thoughts of peace and not of evil, to give you a future and a hope." To leave no doubt, God reassures us that He wants the best for us, peace and a future with Him forever.
No matter what problems we face, God has a glorious end-game in mind for us. Christ points to it in Luke 12:32 as a reason not to fear: "Do not fear, little flock, for it is your Father's good pleasure to give you the kingdom." The end-game for this physical life is only the beginning of the next—eternal life. Our God, with all the power at His command, is committed to getting us there, as I Thessalonians 5:23-24 reveals: "Now may the God of peace Himself sanctify you completely; and may your whole spirit, soul, and body be preserved blameless at the coming of our Lord Jesus Christ. He who calls you is faithful, who also will do it" (emphasis ours).
Our God is not passive in His love for us but is actively looking for opportunities to do us good, assuring us in II Chronicles 16:9, "For the eyes of the LORD run to and fro throughout the whole earth, to show Himself strong on behalf of those whose heart is loyal to Him." Not only is it God's will to be a present help in trouble (Psalm 46:1), this verse in II Chronicles also reveals that He takes it much further. God is with great effort, illustrated by running to and fro, actively looking for opportunities to help us. Kiel and Delitzsch says of this verse, "[He] looks forth over all the earth, uses every opportunity wonderfully to succour those who are piously devoted to Him."
So knowing that God is looking for every opportunity to help us, we should be able to relate to Psalm 121:
I will lift up my eyes to the hills—from whence comes my help? My help comes from the LORD, who made heaven and earth. He will not allow your foot to be moved; He who keeps you will not slumber. Behold, He who keeps Israel shall neither slumber nor sleep. The LORD is your keeper; the LORD is your shade at your right hand. The sun shall not strike you by day, nor the moon by night. The LORD shall preserve you from all evil; He shall preserve your soul. The LORD shall preserve your going out and your coming in from this time forth, and even forevermore.
With all these promises, why do we have fear? We will fear if we do not make God's promises part of our thinking or lack the faith to believe them.