For the same reason that people look up when you look up. You can call it the power of suggestion.
its to do with the mide to find out more i would proberly go on health websites or if your that conserned go to doctor or your actual GP
yawns are very contagious. so when we yawn we got it from someone and gave it to someone else.
Because when you see some one yawn that makes you sleepy and you yawn unless you hold it in
We yawn because it it a signal from our bodies telling us that we are tired. Yawns are not really contagious, it is just a myth.

I've heard you yawn because your body is telling you that you need to keep breathing! it is because your breathing slows and your body catches up, unless it is forced yawn.
Have u ever realised that u smile when someone else smiles at you.... similarly you yawn when you see someone else yawning.This is a stimulation reaction by you stimulii....this action is stored in your brain....you yawn every time you see someone yawning and also when you feel sleepy or drowsy.....when you feel either sleepy or drowsy. Your respiratory system starts taking in more of carbon di -oxide and less of oxygen...when your brain realises this it commands your body to yawn in the process more of oxygen is taken in to reduce the large amount of corbon di oxide in the body.Therefore you yawn every time you see someone yawning and also when you hear,say or read the word 'yawn'...i bet as you are reading this you must be yawning!!!
Yawns can be very contagious. Even just thinking about yawning can make you yawn. I actually started to yawn answering this. Its just your brains way of working. The scientific answer is due to minor neurons in your body.
When someone yawns and you yawn it is contagious but unharmful daily thing in your life.

Studies showed that your brain isn't getting enough oxygen so you yawn and when someone else yawns your brain actually thinks theyre taking your air.
One states that yawning occurs when one's blood contains increased amounts of carbon dioxide and, therefore, becomes in need of the influx of oxygen (or expulsion of carbon dioxide) that a yawn can provide,[5] but studies have since shown it to be either incorrect or, at the very best, flawed.[7] Yawning may, in fact, reduce oxygen intake compared to normal respiration, not increase it.[8]

Another speculated reason for yawning is the desire to stretch one's muscles.[9] Yawns are often accompanied by the urge to stretch. Prey animals must be ready to physically exert themselves at any given moment. Muscles required to do this function more effectively when stretched, which is why human athletes stretch before exercise. Considering the increased amount of oxygen the body needs to sustain physical activity, it may be that, to perform best, the lungs and throat muscles need to be "limbered up" as well. Therefore, yawning could be the body's way of helping to keep the body ready for action. There have been studies that suggest that yawning, especially psychological "contagious" yawning, may have developed as a way of keeping a group of animals alert.[10] If an animal is drowsy or bored, it may not be as alert as it should be to be prepared to spring into action. Therefore, the "contagious" yawn could be an instinctual reaction to a signal from one member of the "herd" reminding the others to stay alert. Nervousness has also been suggested as a possible reason. Nervousness often indicates the perception of an impending need for action. Anecdotal evidence suggests that yawning helps increase the state of alertness of a person-paratroopers have been noted to yawn in the moments before they exit the aircraft
because you both have different bodies working
I don't.
It is contagious
no one knows

Improving answer: It is theorized that when we were still apes that yawning was a way of signaling the rest of the group that it was time to settle down, relax, take a nap.

People have a type of Hormone called Melatonin.
This hormone promotes sleep, which means it makes you tired, allows you to sleep.
Melatonin is found in everyone, specially teenagers, they have an extremely high ammount of melatonin.

What I think is going on, is that when a person yawns, the melatonin gets released and spread through the air, hitting someone else with low amounts of melatonin, and promoting them to get tired.
Because you want to do the same thing as the other person

Edit: There is an actual scientific reason as to why we do this. Here is the direct order of which the action takes place:

1. Person observes yawn

2. Sensory input of the brain deactivates left periamygdalar region

3. Subject yawns in response.

It is mainly social empathy in which causes this reaction to viewing another's yawn.

Hope this helped.
yes, because it is a natural instinct for people to yawn when others do! i heard that cave men didnt know how to talk so they would comunicate by showing there teeth. if you yawn you see there teeth. right? so this is why you yawn when you see someone else yawn!!!

Edit: There is an actual scientific reason as to why we do this. Here is the direct order of which the action takes place:

1. Person observes yawn

2. Sensory input of the brain deactivates left periamygdalar region

3. Subject yawns in response.

It is mainly social empathy in which causes this reaction to viewing another's yawn.

Hope this helped.

One potential explanation comes from research out of the University of Nottingham, which suggests that contagious yawns are prompted by automatic, primitive reflexes in the part of the brain responsible for motor function.

Other researchers think it has something to do with empathy or social cues.

"Researchers have seen that yawning may not be as contagious to people with autism or schizophrenia," Meredith Williamson, clinical assistant professor at the Texas A&M College of Medicine, told Vital Record. Individuals with these conditions can struggle with empathy.

A 2013 study did find, however, that children with autism experienced contagious yawns as often as non-autistic children when made to focus on the yawner’s face, suggesting social cues, rather than empathy, are at play.

Edit: Another possible scientific reason as to why we do this:

1. Person observes yawn

2. Sensory input of the brain deactivates left periamygdalar region

3. Subject yawns in response.

It is mainly social empathy in which causes this reaction to viewing another's yawn.

Also: When you see someone else yawn, it's because their brain thinks they are low on oxygen so they open up and yawn to get more. Then, your brain thinks that person is stealing your oxygen and sealing it off from you, so your brains reaction is to yawn and the chain goes on and on and on...

Hope this helped.

Yawning It is actually still an unknown mystery of life. Though there are good hypotheses supporting why humans and animals yawn, they are not conclusive.

Any explanation about why we yawn will have to explain a number of interesting observations:

• Yawning occurs even when oxygen and carbon dioxide levels are normal.
• Yawning may make others yawn (that is, it seems "contagious") and may indicate behavior based on social cues.

• One is that when we are bored or tired, we just don't breathe as deeply as we usually do. As this theory goes, our bodies take in less oxygen because our breathing has slowed. Therefore, yawning helps us bring more oxygen into the blood and move more carbon dioxide out of the blood

  (This theory could be true, because I've never yawned while playing sports or running a mile before.)

• Most people think it is that you are tired or there isn't enough oxygen coming to the brain. From recent studies it is because your head is over-heated.
• Another person noticed that he had been sitting for about half an hour being bored and dreamy. When he stood up, he started yawning 2 or 3 times. Maybe his body was doing the opposite of maintaining homeostasis, preparing for an increased heart rate when his body stood up, as if about to start moving and needing oxygen.