This is a good question and we are not entirely sure of the answer.
It may be that it would have benefited our ancestors (Cavemen) in the past. For example it would benefit the group when hunting if everyone went to sleep at the same time so that they were all alert for the next days hunt. So when one person yawns it signals time for everyone to go to sleep. Obviously this is not really beneficial these days though.
It is partly due to a built in reflex, as @Catherine said. It is also inpart due to awareness. Your are not normally made aware of your bodies state of tireness, however, when someone yawns it is brought to your attention, this causes you to yawn to reduce this.
A yawn is a reflex consisting of the simultaneous inhalation of air and the stretching of the eardrums, followed by an exhalation of breath.
This is a nervous reflex of our brain, just like when someone tells you that he/she is hungry, even if you’re not, you’ll suddenly feel hungry as well.
we tend to yawn when we see someone else yawn because we somehow connect to that person’s mind when he/she’s not having right amount of oxygen in his/her body
Another possible scientific reason as to why we do this: 1.Person observes yawn 2. Sensory input of the brain deactivates left periamygdalar region
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