If you want your facial hair to thicken up, use electric sheers to trim it instead of shaving, massage the skin regularly to increase blood circulation, only use soap when necessary, keep the skin hydrated, make sure you're not missing anything from your diet.

The direction you brush or comb will change how full it looks as well.

Genetics does play a part but unless you're from an ethnic group with low to no body hair environmental factors play a bigger role.