This is an age-old question, one that I have never really seen a clear cut answer to. I think there are many possibilities: they like the taste of grass, they are hungry, they are not feeling well and eating something to see if that helps (kind of like humans do sometimes!).
Grass is also part of a carnivore's diet -- usually consumed when they feed on smaller prey and consume the entire animal; including grassy stomach contents.

Most dogs do best with 2 or more small feedings a day rather than one large feeding. Cats optimally would feed several small meals throughout the day. Ideally, for both dogs and cats, food should be available at all times. This aids in weight control (food is no longer a "big deal" and helps maintain digestive health. This is not possible in many situations and busy lifestyles, but if possible, my preferred way to feed pets.

A commercial diet or well-prepared home diet should not be lacking in essential nutrients. If they have an empty stomach, bile may reflux (flow back up in to the stomach from the intestine) and this is irritating. This can cause vomiting of clear, yellow fluid.

Any time that your pet experiences lack of appetite, eating odd items (pica), or vomiting, it is always best to check with your vet. Gastrointestinal blockages are emergencies. Changes in appetite or vomiting can also indicate internal disease (kidney failure, hyperthyroidism, etc.), toxin ingestion, or infectious disease, just to name a few possibilities. Always best to check things out with your vet.

By Janet Tobiassen Crosby, DVM, About.com

Patchs eats grass a few times a week sometimes. Usually it leads her to vomit. The Vet has no explaination or suggestions. She never looses her apetite and is back up and running in a few minutes.

As a precautuion, I usually skip a feeding or give her only a small amount.

I personally dont beleive that open feeding is a good idea. If I did that with my dogs they would be so far over weight that they would look like beer kegs with legs. This was also the case with Chip. Insteed, i feed an ample amount of food 12 hours apart.

I agree with you there, I'm not a big fan of free feeding myself for the specific reason that Dr. Crosby mentioned "food is not no longer a big deal". Some dogs that are free fed actually aren't as motivated by food treats...now this could be a good thing, but generally it isn't.

The vomiting most likely happens (and I'm surprised it wasn't mentioned) because the grass causes the stomach to become irritated, and of course they vomit it. Some dogs will pass it through though. The FAQ on K9Domain has my take on why dogs eat grass.

Steven_L wrote:

The vomiting most likely happens (and I'm surprised it wasn't mentioned) because the grass causes the stomach to become irritated, and of course they vomit it. Some dogs will pass it through though. The FAQ on K9Domain has my take on why dogs eat grass.

Patchs "sometimes" doent need the grass ...

But when she eats grass, it only takes about 30-45 mins for her to purge.

I know what you mean...I've had to pick up after Junior after he's munched on the grass. I hadn't noted the time though...and I know for sure it doesn't happen every time he eats grass.

weird that they keep eating stuff, like grass even if it makes them sick.

My dog eats grass all the time, he loves eating grass. My dog has never gotten sick because of this though.

Well it doesn't technically make them sick, its just that it helps them clean their digestive track and if it makes them vomit then its helping them get rid of anything they have in their stomach.