what's the meaning of だもん


Yamato and Takeo are discussing why their friend Suna is acting strange lately
and Yamato suggests that Suna may be lonely.

I heard this expression a lot lately and was wondering what it could mean.

[![enter image description
here](https://i.stack.imgur.com/SFowv.jpg)](https://i.stack.imgur.com/SFowv.jpg)

[![enter image description
here](https://i.stack.imgur.com/NcsYT.jpg)](https://i.stack.imgur.com/NcsYT.jpg)

