I think fear serves a number of purposes, in healthy doses. As mentioned, it serves as a self-preservation method. A self-proclaimed fearless person will be more likely to find his/herself in a threatening situation than one who has a comfortable understanding of fear. Fear is a weapon, a tool, a tactic, a method... used in so many ways for both benevolent and malevolent purposes. Fear can keep a nation under the rule of a tyrant as well as keep a young child from going off with a stranger. Understanding how one should use and live with fear is a journey, i think.
A person who finally reaches a level of mental maturity to understand their own mortality will likely fear it first. I know i've spent countless sleepless nights simply pouring over the idea that even I will die. My body, my personality, the importance of my memories, my cognitive thought processes will cease. Then what? Without being alive i'll never know that answer and not knowing is the scariest thing of all. It's because of this fear of death that keeps me from acting recklessly, taking extra precautions when necessary.
Every has their own level of fear and phobia. There are perhaps people who have a healthy level of fear, enough to keep them safe by instinct yet no phobic response (is phobic even a word, if it ain't, it should be. I can dig it!) to any triggers. Others have the same fear but respond irrationally to certain triggers, spiders, clowns, heights, toasters (it's a real phobia.. i mean think about it, you never know when the toast is going to pop up, and you watch it so it won't startle you, but the harder you watch, the more it DOES scare you... toasters are evil). I honestly can't say that i know how phobias work, if they're an obsessive fear response, or something entirely different, but the average phobia does involve something that is potentially very dangerous. Spiders can be poisonous, Clowns.. one word, guys... Pennywise. Heights, falling hurts alot and the higher up you fall from, the more it hurts. Although the response is usually irrationally extreme, it still serves the basic fear purpose.
As for love, well... rejection gets you thrown off the island and has since the dawn of man. it's simply another self-preservation tactic i think.
That's what i think about it, anyways. Mama always said i wasn't an especially bright child 😀 .