In Astrology, Mercury represents communication, while Jupiter represents expansion, optimism, and good will. Mercury represents the lower mind, while Jupiter represents the higher mind.
Harmonious aspects (conjunction, trine, sextile) between Mercury and Jupiter in synastry indicates that the two of you intellectually stimulate one another. Jupiter expands Mercury’s communication, while Mercury puts Jupiter’s grand plans and ideas into words. Jupiter is enthusiastic about Mercury’s ideas, and encourages Mercury to learn more. Conversations are lively and expansive, and you truly enjoy speaking and sharing opinions with each other. The Jupiter person often takes on the role of the “teacher”, while the Mercury person takes on the role of the “student”.
Hard aspects (square, opposition, quincunx) between Mercury and Jupiter in synastry also indicate lively intellectual exchanges, but problems can arise. You may tend to compete for the microphone, and may tend to interrupt each other in order to get your ideas in. Jupiter may feel that Mercury is overly-logical, while Mercury may complain Jupiter comes across as condescending and idealistic. The two of you may have different philosophical views.