Most Helpful Guy
Some reasons are that they want to know what is wrong with you because many think being single is wrong and being in a relationship is right. And being single by choice is beyond the comprehension of many people.
In some cases it can also be a backhanded compliment where they are saying that you are a great woman and therefor it's odd that you are single. Once again the perspective than being single is wrong.
Most Helpful Girl
not really sure why. Not quite as bad as '... you're STILL a virgin? :o '
As if there's some sort of cut-off date after which, if you're still not sexually active, you're past your shelf-life or something. -_-