Option 1: They look in mirrors so much the whole reflecting light thing was just second nature to them.
Option 2: During the Changing process from human to vampire someone thought it would be funny to drop a mirror on them and all the little bits got stuck when their skin went hard and cold.
Option 3: They wanted an excuse to get every single sunny day off of school and/or work. They don't really sparkle. They lied.
Option 4: They weren't happy with being stronger, faster and more beautiful than us humans so they decided they wanted to sparkle too.
Option 5: It wouldn't have been as dramatic in twilight if Edward had stepped into the meadow and Bella had said "You have man boobs." So they use the whole sparkling thing to disguise any flab they may have.
Option 6: They hate being so pale. The sparkling covers it a bit.
Option 7: They can earn a fortune by selling random bits of their skin when it's really sunny.
Option 8: They love disco but it has, unfortunately , gone out of fashion. So they hang Edward from a tree and set him spinning on a sunny day and have a proper rave.
Option 9: Blood has Iron in it. Iron is a metal. Metal is shiny. The vampires drink so much blood that the excess iron has nowhere to go. It gets shined up by their digestive system and floats to the surface of their skin and shines like there's no tomorrow.
Option 10: Stephanie Meyer got really bored and decided to write a series of books about sparkly vampires. Yay!