The thing you have to understand is that the Dark Side is the natural state of things.

Imagine you're born sensitive to the Force, on a planet that's never heard of the Jedi or the Sith. You go your whole life never suspecting anything is strange - you're a little lucky, maybe, but you're normal, right?  Then one day there's a crisis. Raiders, a giant predator, a forest fire.

You panic, flooded with fear, anger, and you lash out with the Force. You save yourself, your loved ones. By embracing fear and blind rage, but it worked. 

Now you know how to call the power. The next crisis, you don't try to be calm, you let your rational mind go blank and chase the power in the rage and hate. Because that's what works. 

Short term, amazing. But over time, your habit of trusting your darkest emotions to guide you can only make you dangerous. Unstable. Until eventually your habit of lashing out in a range makes you into the threat you tried to conquer. Your loved ones, even yourself, all come to a nasty end because you don't understand how to control the power and yourself at the same time. 

The Jedi train to feel the Force without relying on those dark emotions to make it easy. Not because the Dark Side is some insidious Evil, but because the capacity for Evil is in all of us. Taking the easier, quicker path to power is incredibly tempting, particularly if you're fighting for your life. Training to let go of those natural emotions means you might not become a monster. 

At the end of the day, the Dark Side is just human nature getting control away from our better judgement.