もの, an extremely common noun in Japanese that means ‘thing’, can also be used as a 終助詞（しゅうじょし） (sentence ending particle). In these cases, it adds a small amount of emotion to a statement, and presents it as a reason. Due to this, もの is usually translated as ‘because’, or ‘cause’. Literally, it just means ‘it’s an (A) thing!’. もの is quite casual, so should be avoided in formal situations.
To use もの as ‘because’, attach it to the end of the 終止形（しゅうしけい） (plain form) of any word.

• あなたが言（い）う事（こと）は何（なに）も信（しん）じられないわ。だって、あなたはいつも嘘（うそ）をつくもの。
  I can't believe anything you say. Cause you always lie.
• だって、彼女（かのじょ）が悪（わる）いんだもの。
  Cause she is the one in the wrong.
• これはあなたにあげられない。大切（たいせつ）だもの。
  I can't give this to you, cause it is important.
• すぐ泣（な）くに決（き）まってるじゃん、まだこどもだもの。
  Of course he will cry, cause he is still a child.

To emphasize もの even further, the explanatory んだ (after a verb or い-Adjective), or なんだ (after a な-Adjective or noun) will often be used.

• エアコンをつけて寝（ね）ているよ。だって、暑（あつ）くて寝（ね）れないんだもの。
  I sleep with the A/C on. Cause it’s so hot that I can't sleep, you see.
• この子（こ）は失敗（しっぱい）するに決（きま）ってるでしょう、まだ新人（しんじん）なんだもの。
  Of course this kid will make mistakes. Cause he’s still new here.