Why does Shakespeare use light and dark as comparisons in "Sonnet 147"?
The speaker is saying that love is deceptive, because in the beginning all you see is the good things and by the time you realize the bad ones you are hooked.
Love seems bright in the beginning. Before passion turns to infatuation, it burns brightly. Everything in the world looks better, cleaner, brighter, and happier. Love turns everything you touch to gold.
However, sometimes love turns dark. We can mistake the darker undertones of love because we are so distracted by the brightness at first.
For I have sworn thee fair, and thought thee bright,
Who art as black as hell, as dark as night.
The use of bright and dark are sharp contrasts, showing us the full spectrum of emotions we can feel with love. Anything at extremes is dangerous. Too be too far to one extreme can actually push us too far to the other—being too much obsessed with the light can make us too far obsessed with the dark.