The reason why Paul is my favourite co-worker is because ...
ーThe reason why it's my favourite is because it tastes so good.
「それを好む理由はとても美味しいからです」
The reason why it's my favourite is because ... で「それを好む理由は…だからです」
ーThe reason why Paul is my favourite co-worker is because he is always willing to help me when I need it.
「ポールが私のお気に入りの同僚な理由は、私が必要な時にいつも喜んで手伝ってくれるからです」
The reason why 〇〇 is my favourite XX is because ... で「〇〇がお気に入りのXXな理由は…だからです」
ご参考まで!