The anti-parallel structure of a DNA helix would not be able to work properly in a parallel fashion. While there are a lot of components at play here it really comes down to hydrogen bonding. Hydrogen bonding is really important and key for the DNA helix to hold its structure. The anti-parallel pattern allows for the strongest hydrogen bonds to form between the base pairs. If the strands were try to form together in a parallel pattern the hydrogen bonds would not form between the base pairs and the entire structure would be unstable. I hope this answer helped. If you have any further questions feel free to send me a message!