To love moderately means to not take their love to extremes and so when Friar Lawrence told Romeo and Juliet told them that, he meant to take things slowly and to be careful. This is good advice because as we all know they kill themselves in the end of the book because they did not love moderately. If they had loved moderately, then they would have not killed themselves. For the sake of the story, it’s good they killed themselves but in real life, it would have been terrible if they didn’t follow Friar Lawrence’s warning. Their relationship started with Romeo taking extremes and Juliet following along, which I personally think is alright, but later in the relationship it would be smarter for them to have taken things nice and easy because if they continued being as extreme as they were before, then it would end violent, just like how it started. And it did end violently.