It's fun because we only see each other once in a while.
We don't see each other all that often, so when we do get together it's always enjoyable.
この場合、次のような言い方ができますよ。
ーIt's fun because we only see each other once in a while.
「たまにしか会わないから楽しいんだよ」
only see each other once in a while で「たまにしか会わない」
ーWe don't see each other all that often, so when we do get together it's always enjoyable.
「しょっちゅう会わないから、会うといつも楽しいんだよ」
don't see each other all that often で「しょっちゅう会わない」
to get together で「会う・集まる」
ご参考まで！
カナダ 
