The Moon is the most important 'planet' in relationships. The Moon represents deep security needs, and it is difficult to be with with someone who does not offer us what our Moon needs. Positive Moon compatibility will show two people who feel "at home" with one another, and instinctively care for one another. It's like being family. That's why it's the most important planet in synastry. If you want to spend the rest of your life sleeping in the same bed as someone else, you obviously need to feel like family to one another.
Moon Opposition Mercury
You will feel very different from your partner in your temperament, upbringing, romantic needs, ideas about domestic harmony, desire for emotional security and your personal tastes. You will find yourselves having seemingly endless discussions over major and minor matters that always seemed perfectly simple and straightforward to you, but are in your partners eyes issues of importance. When the Moon person is in opposition to the Mercury person, you will find yourself questioning the things they says and the reasoning behind their complacency with such a narrow point of view. That's what attracts you two, you are together to have long conversations and deliberations in order to gain perspective and awareness of ideas and approaches you never would have imagined on your own.