This is a very large question, so I will try to get you started and hope to steer you in the right direction.
A main point of Plato's Symposium is that everyone has a perfect other. If one can find that person, the unity of the two leads to a perfect wholeness, reuniting an originally perfect form that has been split in half.
A main point of Erich Fromm's The Art of Loving is that love doesn't just happen but has to be worked for.
If we put Plato and Fromm together, we can see two things that are important to Far From a Madding Crowd: everyone may have an ideal mate and yet, at the same, time, love has to be worked for and worked out: one's ideal love may exist somewhere in the universe, but that doesn't mean true love is simply going to "happen" effortlessly and fall out of the sky and into one's lap.
The misery in Hardy's novel can be seen as the result of the wrong love pairings. Gabriel and Bathsheba are meant to be together, as are Troy and Fanny. However, largely because of accident (always important in Hardy) and class difference, the pairs of lovers who are "meant to be" in a Platonic way fail to unite. Troy misses marrying Fanny when she goes to the wrong church. Bathsheba initially turns down Gabriel's marriage offer. After a disastrous mishap leads to Gabriel losing his sheep flock and becoming an impoverished employee of Bathsheba, the two can not think about marrying. Troy disastrously misses marriage with Fanny, leading to her death, and disastrously marries Bathsheba, a woman he doesn't love, for her money. In a Platonic universe, these misfires cannot lead to happiness or harmony.
Moving to Fromm, we could understand these mismatches as a result, too, of characters who want love to come too easily and fall into place without effort. Troy is too quick to give up on Fanny and too quick to marry Bathsheba. Bathsheba is also too quick to marry a dashing, romantic soldier who is not right for her. Troy deeply regrets Fanny's death, while Bathsheba cannot find happiness until she can do the work of recognizing that Gabriel, not Troy, is the man with whom she is truly meant to share her life.