Weight loss: why does my weight decrease and increase again as I try to lose it?

Balance. The body likes balance. It might think your current weight is the ideal weight and will "readjust" to keep you at the current weight. The goal is to "trick" the body through small weight loss (1-2 lbs /week maximum) so that you can maintain the pace. Anything more than this and the body will be alarmed and go into its compensatory mode.
Loss of muscle. Yoyo weight changes are due to muscle loss. When restricting calories, the body will lose muscle. To prevent this, you need to eat protein during weight loss w/calorie restriction and increase water dramatically which will help decrease fat. See a physicain who can monitor your muscle/fat/water ratios to help keep this in check. Also brain needs 2 years to reset the weight at lower levels.