Because it feels good.
Seriously, think about it like this: You just found out you have superpowers. You realize you have two, *maybe* three, choices. You can A) Enter a monastery, distance yourself from your friends and family, avoid any romantic relationships for the rest of your life, and try to cultivate a nebulously defined "inner peace" to safely develop your powers. Or B) follow your instincts and embrace your emotions to fuel and direct your powers without having to sacrifice anything. The only problem being that you are told that this will eventually lead you to become a horrible evil person.
Now think about it, if someone told you this how much would you *really* believe that you could become an evil monster just because you got some cool powers and refused to become a monk. Surely you would never do any of the crazy, horrible, monstrous things you've heard about a few psychos doing, that's not who you are right? So how dangerous could it really be to keep living your life like you always have, your feelings are what make you human after all, right?
Besides, no one said you had to run off and join the Sith. You can always C) just find your own way. Maybe let the Jedi teach you how to use your powers, but once you've got what you need they can't make you live like a monk, they'll just kick you out of the order. No big deal there, you don't really want to live that life anyway. It's not like you're going to suddenly start murdering or torturing people, you just want to go back to living a fulfilling and enjoyable life of your own.
That's why people take the easy road. Either right away, or once the constant discipline and self-restraint begins to become intolerable. While some people manage to stay "Grey", a lot end up taking the very tempting road all the way down, because it starts with small things and snowballs into bigger things before you catch yourself. You can see a number of dark-side force users who weren't nice people, but weren't looking to blow up random planets for no reason either.
You don't have to go full-on evil to become a powerful dark-side force user, but after a while it can be hard to remember why you aren't committing atrocities for kicks if you let yourself get carried away or end up under the influence of someone who enables you and pushes you to bigger and badder exploits.
There's a lot more I could say on this, but it's largely an expansion of the above.
I can tell you this though. If I became a force user, within a year I'd be fully aligned to the dark-side. Not by choice, but by simple nature. I can't let go, I can't detach from my feelings and instincts, and I will give into my passions. Simple as that.
All hail the wise Force-councilor, for he knows all about the Force and its balances!!!