Fears can be very real in our lives. On the outset, they seem irrational though. One of the ten most common fears is the fear of clowns, which happens to be my son's fear. He would steer clear of them. It does look irrational to others, but for those who has the fears, nothing can be more real.
What causes fear?
The way I see it, they usually arise out of past experiences. My son remembers when he was still small sitting in stroller being brought near against his will to the most hideous moving thing around, a clown. He must be only 2 or 3 then, but he remembered it. The odd thing is, even after you identified the cause of the fear, it does not get alleviated. It can even become worse.
How then can we overcome our fears?
I have nothing to give other than what is already given in Ephesians 1:13-14: And you also were included in Christ when you heard the word of truth, the gospel of your salvation. Having believed, you were marked in him with a seal, the promised Holy Spirit, who is a deposit guaranteeing our inheritance until the redemption of those who are God's possession—to the praise of his glory.
And the old faithful Isaiah 41:10:
So do not fear, for I am with you;
do not be dismayed, for I am your God.
I will strengthen you and help you;
I will uphold you with my righteous right hand.