Question

positive charge on double bond is unstable why​

Answer 1

Carbocations or carbenium ions in which the positive charge is on a carbon atom, are unstable.

A positive charge on carbon frequently makes a molecule reactive

Answer 2

carbocation are  carbon  atoms  in  an organic molecule bearing  a positive  formal charge  therefore they are carbon  cations have only six electrons in their valence shell  making them  electron deficient thus they are  unstable  electrophiles and will react very  quickly  with nucleophiles to form  new bonds