Some people react badly to lasik eye surgery. why is that?

Many reasons: There are a ton of reasons why lasik complications can occur ranging from being a poor candidate in the first place (something about you or your eyes that made you not a great candidate for the surgery) to intraoperative complications to simple bad luck. Remember, no surgical procedure has a 100% success rate.

Bad luck, bad choice: Like all surgery, lasik has risks. Fortunately under ideal circumstances they are very rare. Measures to minimize risk include avoiding patients with abnormally thin or irregularly shaped corneas. These are easily found with testing. Most of the complications of lasik involve the flap that's made. Other methods, like prk, avoid the flap and are safer.