Only because
Used to emphasize that something is true solely due to the reason given before it.
Examples
彼の努力があったからこそ、この成功を手に入れたのです。
This success was achieved only because of his efforts.
君と一緒にいるからこそ楽しい。
It is enjoyable only because I am with you.
人を助けるからこそ、幸せになれる。
You can become happy only by helping others.
Quick check
人を助ける___、幸せになれる。