Why do girls have to overanalyze everything?

Because we don't want to miss something.

The five answers to this so far are pretty good.
First I'd like to make the distinction between obsessing and analyzing.

Analysis (good)= separation of an intellectual or material whole into its constituent parts for individual study.
Obsession (bad)=The impulsive preoccupation with a fixed idea or an unwanted feeling or emotion, often accompanied by symptoms of anxiety.

Analyzing is bad when it is not serving its purpose... which should be to make good decisions and make your life better. If too much analysis is paralyzing your decision making you need to reevaluate that process. Here is a really good article on decision making
http://humanresources.about.com/od/leadership/a/decision_making.htm

As far as relationships are concerned, here's my take on it. Girls analyze to protect themselves. They often do not have the muscles and resources to protect themselves or seek revenge if they are wronged. A little fore thought can keep them safe. When women make excuses for bad behavior by men or other people and they do not make an educated decision they end up getting hurt (financially emotionally etc...). People will accuse others of over analyzing, this happened to me yesterday so I decided to do some research. People will use over analyzing as a scapegoat so they do not have to deal with an issue or a problem, and it is not just men that play this card. If there is a red flag see it as a red flag and don't think it's going to be ok, educate yourself before jumping to a conclusion. Good rule of thumb (and I got part of this off another blog) if you are taking more than 10 minutes to pick out the restaurant you want to eat at you are over analyzing. If you are only taking three hours to evaluate something that has shaken your CORE values and morals you are under analyzing. An example of over analyzing, if you were to find out pens don't work in space and instead of using a pencil, you spent hundreds of hours and tons of money to make a pen work in space you would be over analyzing. Always look for the simple answer.

As for people accusing people of over analyzing if you have a brain and your partner doesn't want to think. Put up with it and shut up and realize that's how its going to be or move on, but don't torture the other person by trying to change them.

As for me, I see my analyzing as keeping me from bad decisions. It just so happens I've been making a lot of decisions about relationships lately. This is the second time this year a man has accused me of over analyzing. (I'm dating -ugh). The first man admitted I was right. The man that accused me yesterday - I think I hit the nail on the head. He just doesn't want the truth. So I may still be alone but do I really want to be with a guy that can't even be honest with himself? How will he ever be honest with me? C'est la vie. I'm ok alone.

We have been lied to so much that we want to know all the facts. If we even sense that something is being held back, we will try to find out what it is and why it is being hidden.

You know- that is a very good question. I guess we just can't help ourselves. We want to know exactly what is going on, what this or that really means, etc.

Sometimes they'll leave out just a little something, especially when talking to guys, hoping that the guys will pick up on their hints. So naturally, they assume that guys do the same thing when they speak. It's a common misconception. However, guys typically say exactly what they think, exactly what they feel, exactly what they want, and they usually leave nothing out. Most women haven't learned this yet.

Women are organizers and social people by nature. They want to know all the facts and will pour over a problem relentlessly. Men are fixers. We identify a problem attempt to fix it immediately (albeit not always the right way) and move on. Its just the way our brains are wired