One should not see the Sun during eclipse because the sudden change in light intensity may damage the eye.
The human eye, as a protective measure, can adjust automatically to varying light conditions – under bright illumination, as during the day, it closes partially and under dark conditions, as in the night, it opens fully. However this process of adjustment takes some time, a few minutes.
As it is dark during a total solar eclipse, the eye is fully open to let in more light.
After totality, as the Sun emerges the increase in light intensity is so sudden that it catches the eye unawares. Being very bright it can damage the retina even in a few seconds.
