Disclaimer: American cultural point of view.
Why? At most, if not all, jobs, you're as worthy to the employer as a chair or a printer. The days of a company being loyal to its workers are long, long gone. Your boss may be sincerely nice to you, but at the end of the day, your person doesn't matter, your output does. Once you CTB, you essentially stop producing revenue for the company. They'll feel sad for a day or two, and they may send a condolences card to your home address, but at the end of the day, what they'll worry about most is how the company will operate with you gone.
So what that means is you don't have to give a two-week notice, or any kind of notice. At most, what you can do is tell a few very trusted colleagues that you're thinking about quitting. If asked why, make up an excuse about not feeling happy working there and/or personal difficulties. That's it! Don't share any further details. Share just enough to give them a comfortable explanation for why you suddenly stopped showing up.