Why does Alison pretend to die in Pretty Little Liars?

Answer 1

she didn't pretend to die, she never died. The one who died was her twin sister Courtney who was mentally ill and pretended to be alison, so in revenge the real alison killed her sister.

Hope this helps because its a little confusing to understand

Answer 2

Alison never pretended to be dead. She found out that her crazy in the head twin sister, Courtney, was pretending to be her. So, the REAL Alison strangled her and pushed her in a ditch that was in the middle of their gazeebo, because it was being reconstructed. Because everyone thought Courtney was Alison, the REAL Alison had to be Courtney. It took a year for the town to find Courtney's body (everyone thinks she is Alison).