The game first nudges you toward the new content when various strangers remark to you that a new group called The Dawnguard is looking for recruits to help hunt down vampires. After the first quest, though, you're given the option to side with the creatures instead.
Honestly, I can't imagine why anyone wouldn't. Especially once they see what being a vampire means: you get to transform into the uber-badass Vampire Lord form.
Along with powers to become invisible, enchant NPCs, raise powerful dead allies, and detect nearby creatures in your normal form, you can transform into the floating, winged monster on a whim and gain a host of additional powers and attacks, plus a whole new skill tree to master.
You'll move faster in this form, as well, although the downside is being out in sunlight stunts your magicka, health and stamina.