I’m only grinning cause I’m supposed to smile
In the situation he is/was at, it’s hard to grin. As previously described in the song, he’s lost, away from home, and he’s so tired of running. He’s in a tough situation, in which he wouldn’t want to grin or smile. But he does grin, only because he feels like he’s supposed to. He feels like everything he goes through is supposed to be fun for a kid because society says so.
To help improve the meaning of these lyrics, visit “Runnin'” by Machine Gun Kelly (Ft. Planet VI) Lyrics and leave a comment on the lyrics box