Why is the DNA helix anti-parallel?
Parallel nucleic acid double strand is possible but it is not as stable as the antiparallel form (Szabat and Kierzek, 2017). This is because the nucleobases are not aligned in a way that is conducive for the Watson-Crick (WC) type base pairing. In parallel conformation, the bases can form Hoogsteen (HS) and reverse Watson-Crick (RWC) type base pairing (see below). 



You can see that these base pairs are not as strong as that in WC base pairing:


No triple bond between G and C in RWC base pairing
GC pair occurs in HS base pairing only when C is protonated at low pH


Formation of parallel helices, therefore depends on the sequence.


  In general, the formation of duplexes with parallel strand orientation
  is determined mostly by the sequence context and pH conditions.
  Fragments of RNA or DNA capable of forming a parallel duplex are often
  rich in A and C, which is related to their ability to become
  protonated, in middle acidic conditions.


However, it is not as simple as RNA/DNA base pairing with its complement. Parallel helices would not follow the WC base pairing rules and therefore predicting whether they will form is not that straightforward. However, parallel helices can form in vivo (see references 23&ndash;25 of Szabat and Kierzek, 2017).

You can also check out this article by Leontis et al. (2002) for hydrogen bond patterns in parallel and antiparallel helices.