The conservative notion of symmetry is that hand fits glove, plug fits socket, but only in specific ways. Hydrogen for example, will not simply saturate a C=C double bond all by its lonesome, e.g.:
H2 + CH2=CH2 ---> CH3-CH3.
The orbital symmetry is all wrong. The chemistry is forbidden.
The reaction requires a catalyst to break the symmetry and to polarize the electorate. How nature really works (at a chemical level) is polarization followed by attack followed by depolarization.