Importantly, molecules do not just bop into each other. These are very regulated reactions. Methylation is such a basic, frequent and important reaction in every cell, that the body created a type of "docking station" called SAMe, the main "methylation factor" which carries and "extra" methyl group. SAMe binds in very specific ways to other molecules to transfer its extra methyl group.
Some of the major reactions that need methylation "factors" incude: 1) clearing toxins; 2) production and breakdown of neurotransmitters; 3) production of energy and cell membranes; 4) regulation of gene expression.
In some people, this "methylation cycle" (the factory of methyl groups) does not work properly. This is why some of the symptoms (of undermethylation) include: depression and internal anxiety, difficulties focusing (or opposite, a little OCD), headaches or feeling "toxic", allergy-like symptoms (lots of histamine), fatigue, hormonal problems and often a family history of cancer or substance use (self-medication).
Important - do not take SAMe by itself by itself, you could have side effects. It is best used as part of a plan that treats the entire cycle and the entire person. Please seek professional advice.