why it is that as you get more intimate with someone, you can tend to not tell her things as much because you are afraid of losing her, your vulnerability makes you afraid.
While I'll certainly grant that it is difficult to reveal so much of yourself to someone you want so badly to impress, I don't think you should give up all hope. There are plenty of guys out there who feel the same way you do.
    ...all (hypothetical SO) wants is someone to not have to think around?

Remember that a relationship is a multifaceted thing. In a good relationship, you connect with your partner on physical, emotional, and intellectual levels. Spending time not thinking with someone is part of the emotional connection. Just because a partner needs this sometimes doesn't mean that's all he wants from you.