John Mayer - No Such Thing Lyrics Meaning

It's about John growing up as a teen, and wanted to be a musician, but his mom forbids him.

the line "they love to tell you stay inside the line, but something's better on the other side" is when his mom told him to be something else, because there is no such thing as being a rock star.

there's actually one version of this song, where John added a special intro, it's more obvious there:

Freshman year,
Mom asks "What do you want to do
when you get out of here?"
I said, "Well, gee, I'm only 13,
but I think I'd like to play my guitar...
be a star."
"Well," she said, "that's not it,
you got time to go.
Good sense will kick in anytime, you know.
I'm not worried, I'm not worried."

Sophomore year,
she asked the same old thing,
my answer had remained unchanged.
I saw her fidget with her thumbs.
She said, "You like computers, John,
you like computers, don't you?
Yeah, I always see you with computers...
you like computers
maybe you could do something with computers.
Yeah, wouldn't that be nice?"

Junior year,
It's a little more intense,
She says, "What do you want to do with your life?"
I'm not on the fence, I know exactly what I aim to be,
and I'm only 17.
She said, "Well, all I ask, is that you pay attention in class,
so if you happen to change your mind in time,
you can still go somewhere reputable...
do us proud."
So I went downstairs,
and I played guitar loud.

Yeah, senior year,
Same old question,
She said, "What do you want to do?"
I said, "Play my guitar and sing."
She said, "There's no such thing."
She said, "There's no such thing..."

(speaking: I love you Mom)