What does yet natures tears are the reasons merriment in Romeo and Juliet?

It makes more sense if you quote it accurately. The quote is "Yet nature's tears are reason's merriment." The apostrophes are essential to a correct understanding of the passage and why there is no Word "the" before "reason's". They tell us that the tears belong to nature, but the merriment belongs to reason. Reason is our ability to think, and is contrasted with nature which teaches us only what to feel. So, says the friar, it is natural to feel sad because Juliet is dead, but if we think about it, we realize that there is something to be happy about here: Juliet will go to heaven where she will be happy, and why should her parents feel sad because Juliet is happy?