Manufacturing joy is easy, yes. Tie a counter to a semi-random process that's semi-controlled by a human and they experience joy when the counter goes up. The so called slot machine. THis is how HackerNews karma, Facebook notifications, your email inbox, and other addictive activities are designed.
You experience momentary joy.
But fear, fear is forever. How many fears have you been dragging on since childhood that never seem to go away no matter how irrational they are? Could've been from a single event like watching a scary movie about a clown, or a case of near-drowning, or ... well there's a lot of common phobias. The list can go on forever.
You always need to make a new slot machine. You rarely need to make a new fear.