why do men like putting down my looks?

hey, i'm ish and i'm 15, turning 16 this year. i wouldn't consider myself 'insecure' however some comments on my appearance really unsettle me. girls my age have called me pretty, so have quite a few guys. however, sometimes, some guys just start dissing my appearance with comments like "stfu fattie" or "you're so ugly". now, these comments don't usually faze me, however they're often very out-of-the-blue. i'm just wondering if it means that i'm actually ugly or if they're just hatin cause they bored.