"I like juice."
は is sometimes a contrast marker. If the sentence were ジュースは好きです, it would imply that you like juice but not some other unmentioned thing (e.g. tea). が does not carry this connotation.
For example: ジュースは好きです。お茶は好きではありません。(I like juice, but I don't like tea.)
Another example: John and Rose are an interesting couple. Imagine someone asks you if you like them. You might answer: ジョンは好きです。The translation would be "I like John," but since you've used は, you're leaving the contrast unspoken, "I like John (but I don't necessarily like Rose)."