Obviously because you are awesome and cool. What about beyond that? If you have ever been curious about the dynamics between your followers and you there's a new (and FREE) site that will let you find out why, exactly, those followers are following you!
Once you join Why Follow it will send out a message to your followers asking them why they follow you. Then Why Follow gives you an editable list of the most used reasons someone might follow you. You may edit this list to include more personal responses.
So why does it matter why people follow you? Well if you know why the follow you, you can customize your content to give them more of what they like and attract more fans. It could also show you if you have any divergent groups within your fans. For example if you have half of your fans that enjoy your witty comments, but half that enjoy your opinions on the latest hottest music, but no one really cares what you ate for dinner, you would know you could focus on those the witty comments and music reviews and cut back on the dinner posts (sorry, dinner was awesome but your fans are just too busy posting about their dinners).
Are you brave enough to ask your fans Why Follow? Have you already asked them, what were the responses? Tell us about it in the comments!