May the (Good Side) of the Force Be With You
Basically, as soon as people stopped eating hunks of half-raw meat out of the fire, they started struggling against temptation and their darker nature. From a practical standpoint, it makes sense: your "darker" nature is where all of the bad stuff lies. It's not fun to be angry, bitter, resentful and fearful. But it is easy.
This struggle is half the reason religion exists: to codify what is "good," what is "bad," and ways to adhere to the rules so that you can consider yourself on the winning side.
So, it's no coincidence that the Force is often referred to as a religion, and its believers as monastic followers … because they are basically indistinguishable. The Jedi side of the Force represents all that is good, while the dark side reprsents bad. Jedi stuff is bright, pure, and heavenly. Sith stuff is dark, smoldering, and smoky.
Without going into Star Wars lore about what the Force consists of (which, let's face it, would take a long time—the midi-chlorian debate rages on), we can definitely delve into what it represents.
The Force, as the Jedi refer to it, is everything that is good about existence, but it is also ubiquitous and binding. Aligned with compassion, selflessness, enlightenment, and mercy, its adherents often also have to be quite familiar with sacrifice and repression of some very human instincts (like falling in love and taking a spouse, for one thing).
Sound familiar? They even wear simple brown- and cream-colored robes, devoid of embellishment, much like some other religious believers who are also sequestered apart from society and unable to marry (cough, monks, cough).
We have to be totally honest: it's not hard to see why anyone would find the dark side more appealing.
The dark side, in contrast to the broader Force, is all about temptation—it's about giving in to your primal urges like hatred, fear, anger, and jealousy. But, it's also about turning weakness into something more constructive. Perhaps for that very reason, it's sometimes considered the stronger side of the Force because of the strength of emotions it packs behind it.
When Yoda is counseling Anakin about his fear of losing Padmé, he says:
YODA: Careful you must be when sensing the future, Anakin. The fear of loss is a path to the dark side.
ANAKIN: I won't let these visions come true, Master Yoda.
YODA: Death is a natural part of life. Rejoice for those around you who transform into the Force. Mourn them, do not. Miss them, do not. Attachment leads to jealousy. The shadow of greed, that is.
ANAKIN: What must I do, Master Yoda?
YODA: Train yourself to let go of everything you fear to lose.
So, you can see what Yoda is trying to say here, but easier said than done, right?
If you've ever lost someone, you know full well how impossible it can be not to miss them or mourn their loss. That's a very natural part of life. So, to try and fight against those instincts goes against everything that feels normal; no wonder Anakin can't do it. The dark side takes those emotions, though, and converts them into usable energy. What's so bad about that?
A lot, apparently. The downside of the dark side is that it not only uses those strong emotions—it lets them control you. Once it's inside, it insidiously grows, drawing upon rage and fear until it has the power to take over. The way Lucas shows this in Anakin, aside from the heavy-handed glowering, is the darkness of his face. His under-eye circles literally get darker as the dark side takes over.
Exhibit A: a screenshot from the beginning of the movie.
Exhibit B: a screenshot from the middle of the movie.
Exhibit C: a screenshot from the end of the movie.
The scenery itself gets dimmer as the film progresses, too. In the beginning, the vast, sweeping opening shots of whatever planet they're on are always bright and shiny, occurring in the cheerful light of day.
Toward the end of the film, we are seemingly stuck in the depths of dusk, with even Padmé's funeral occurring in darkness. This wasn't accidental. The darkening imagery is a tool to communicate almost subconsciously to the viewer exactly how far the universe has succumbed to the dark side.
And, to complete the religious analogy, picture the final battle on Mustafar between Obi-Wan Kenobi and Anakin Skywalker. It's no coincidence that the planet is covered in erupting molten lava: in the pits of hell, our two protagonists are battling for Anakin's soul. And, what allows Obi-Wan to finally end the epic duel? He gains the higher—dare we say, moral?—ground. Boom.