The number one reason is… Bad lighting!
Sometimes you can control the lighting, and sometimes you can’t – but when you can control it make sure you’re always facing the light full on.
And avoid having photos taken when the light is overhead – whether it’s the midday sun or overhead artificial lights. This gives unflattering shadows to everybody!
Secondly, you need to know your best angles. And have some go-to poses where you know you always look good.