Because She Was Attached To Her Hair...Duh:

Paltrow's new shorter haircut was an emotional transition for the star, because it involved cutting off hair she has had since before her father died.
The actress began growing her hair before film maker Bruce Paltrow died in 2002, and she has since married Coldplay rocker Chris Martin and given birth to their two children.

Paltrow tells the July issue of Harper's Bazaar, "I was very, very attached to my hair. I grew it when I was pregnant. I still had hair from when my father was alive. I made it like a talisman. And then I was ready to let go of it, and so I chopped it off and ... aaah!"

She really didn't chop it all of a-la Rihanna though. She's a little over-dramatic. It grows back...it's just hair.