The Force multiplies everything. It's like being high on drugs. It fills you up with life force and magnifies your ability to perceive the emotional content of everything.
Meditate, and get peaceful, and feel at one with the universe, and you get *really* peaceful and *really* at one with the universe, to the point that you can endure incredible pain and sense split-second motion with sharp clarity and all that crap. Do something good for the right reasons and the feeling of satisfaction of being good is immense and incredibly powerful -- Jedi who lose themselves in this reverie and really let themselves stay in the zone can act on autopilot for days, just following their instinctive "This is the Right Thing to Do" intuition. An ordinary good person has a *bit* of peace and harmony and all that crap and that's why they seem nicer and happier than plain old un-good jerks, but the Jedi have that times ten thousand.
Spin it around in the opposite direction for Sith. Ordinary guy kicks a puppy? He feels a little tiny adrenaline rush, but it fades. Sith kicks a puppy? The sense of overwhelming power and joy at having exercised power against another living thing is enormous. He can feel other people's pain, and if he lets himself he can convert it into pleasure. Ordinary jerk might laugh a little bit to see someone he doesn't like trip and fall -- Dark Jedi *feels* the pain, and where Light Jedi feels the pain and shares it and finds it intolerable and must go help the poor guy, Dark Jedi feels the pain and revels in the fact that it's someone else's and is filled with orgasmic spasmodic twitchiness over it. Before you know it all that twitchiness is manifesting itself as deformed skin and maniacal laughter and blasting shooting lightning bolts coming out the wazoo.
For Jedi, ordinary emotional experiences are like mainlining drugs. You can mainline good, happy drugs that make you peaceful and calm, or bad, nasty drugs that make you scream and smash things. But keep a "balance" between the two? Nuh uh. It's like shooting up meth and then shooting up heroin and hoping that because one's a stimulant and one's a narcotic they'll cancel out and leave you a normal person. Doesn't really work.
(Note that I think the mistake of the Jedi Council wasn't in trying to shut out the Dark Side, but trying to shut out everything that could *lead* to the Dark Side as well. Luke never embraced the Dark Side, but he embraced emotion while not letting the emotion serve as a conduit to the Dark Side. In other words, he was capable of blissing out on shrooms indoors while still going outside once in a while for a jog, without needing to shoot up meth to give himself the energy to do so.)
(I apologize for the non-family-friendliness of this metaphor. I am 100% clean in real life, I swear.)