The Mymble has many children, thirty seven to be exact.
Most of them are loud, most of them are rowdy, most of them keep the Mymble's attention.

Most is not Snufkin.

Snufkin is a quieter one who would prefer keeping his space over being in a crowd.
It's not that he doesn't like people, he just can get overwhelmed by them.
He feels ensnared by them sometimes.

Snufkin has tried so many times to just get some solitude in this house, even resorting to the closet once or twice, but the noise outside of it just always reminded him of how overwhelmed he felt.

He felt like he should be invisible.

But he still would like to be heard when he tries to talk, it's just hard to be heard when there are a bunch of other little voices calling out for attention.

Snufkin slowly stopped trying to get the Mymble's attention knowing that he probably couldn't, or if he did one of his many siblings would take the spotlight, rendering Mymble to only be half paying attention, if at all.

He feels like he might as well be invisible.

He didn't realize he actually would become invisible.

It wasn't a quick process, it never is, but slowly Snufkin turned invisible.
At first he didn't even notice until one day when he looked down at his hands and noticed them almost completely gone.

He supposed he got what was coming.

Snufkin figured it would be better to be invisible, then he wouldn't have to worry so much about people bothering him, but he soon found out that it wasn't so wonderful.

His siblings started running into him more often and personal space seemed to become a bigger issue. He increasingly found it harder to talk to his family when he needed to, whether it was due to them not noticing him, or he just couldn't get the words out anyways.

That is the point, when Snufkin just gave up.

He turned completely invisible and grew quiet.

He actually couldn't talk.

He pretty much just wasn't himself anymore.

He didn't have the urge to try to avoid people.

He didn't have the urge to play his harmonica.

He just, sat there, looking out the window.