Used to convey a sense of justification; 'because', 'that's why', 'it's just that'.
Long explanation:
The ~んだもん grammar point is used when someone wants to give a reason, justification or explanatory information in a more casual and emphatic way. It can be translated as 'because', 'that's why', or 'it's just that' in English. The formation differs depending on whether it is used with a verb, い-adjective, な-adjective, or noun.