The media strengthened the belief
The media with some of its movies have strengthened the belief about the bad luck associated with breaking mirrors. As a result some of the people who weren’t aware of the origin of the belief might have thought that it’s true.
Early people thought that reflective surfaces were magical
Before mirrors people used to think that reflective surfaces are magical. The fact that people could see their reflection in a pond might have promoted such a belief. Because those surfaces were believed to be magical the superstition about breaking mirrors could have been born.
Mirrors were believed to have magical powers
Some early people believed that mirrors can give an insight to the future and so breaking one might lead to misfortunes.
Reflections were believed to have the soul of a person
Some early people believed that a reflection represents the soul of the person and that any bad thing that happens to the reflection can affect the human being.
The Romans believed it takes 7 years to renew a life
The Romans believed that it takes 7 years to renew a life and that’s why the belief that 7 years of bad luck can happen if a person broke a mirror. According to their beliefs after the 7 years the person’s life would be renewed and the curse would be broken.
To prevent people from breaking them
Some people believe that the Myth originated because Mirrors weren’t cheap and easily broken. The fact that people wanted to keep their mirrors undamaged might have led to the rise of the superstition. (See Why is opening an umbrella indoors considered bad luck?)
It was believed that a soul could get trapped inside the mirror
Some early people believed that a person’s soul could get trapped inside the mirror if the mirror was broken. This belief came from the other belief that people’s souls lived inside reflective surfaces.
7 Years are enough for a belief to prove itself true
According to psychologists a person who believes in bad luck might become focused on all bad events that happen to him. Because 7 years is a very long period of time so many bad things are likely to happen to a person during them and this could be one reason the belief got stronger.
Broken glass might have been fatal
Stepping on broken glass long ago might have been fatal and this could be one source of the myth. As many bad experiences happened with broken mirrors people might have started to believe that they are associated with bad luck. (See Why is spilling salt considered bad luck?)
Mirrors were believed to be methods of communication with Gods
Some early people believed that mirrors were means of communication between humans and Gods and thus breaking a mirror could have became associated with bad luck (see why walking under a ladder is considered bad luck).
People Couldn’t really understand them
Some common people could have failed to understand how mirrors work and a result they might have developed some superstitious beliefs about them.
Why is breaking a Mirror considered bad luck
Why is it bad luck to break a mirror? Superstitions busted: Breaking a mirror