Why does a wave-front need to be preserved (for light)?

[Freespader]

We are finishing up our unit on light in physics, and I was a little confused about refraction. What I want to know is why it happens. My teachers says that a wave front needs to be preserved for light, and to do this when transmitting through materials requires a change in direction. However, what I don't understand is why a wave-front has to be preserved. Why does the light "want" to stay in phase with the surrounding light? If you can explain it, that'd be great. Thanks.

 

[granpa]

http://en.wikipedia.org/wiki/Huygens–Fresnel_principle