If Mercury is about analytical break down of concepts, Jupiter (which rules over the signs opposite of Mercury and so, in essence, is the opposite energy) is about the bigger picture. It is about the largeness of vision that fills your heart with childlike enthusiasm about life.
The message it broadcasts as it goes around the world is : "Who knows what's possible? Let's try and see. Let's get to know this place called Earth? What's it all about?".
If Mercury is about collecting facts and information, Jupiter is about distilling that information into a coherent set of ideas. Mercury is the short form, Jupiter is the long form. In other words, Mercury is the daily news while Jupiter is the ancient wisdom and the ecclesiastical knowledge.
Mercury breaks down into smaller pieces and studies them inside out (Virgo, the details, the craftsmanship). Jupiter focuses on understanding how the whole works. Hence the association with religion and philosophy.
Zooming out inevitably leads tot he inclusion of foreign lands and peoples. It suggests choosing to unsee the minor differences and to focus on larger themes and underlying commonalities. Cosmopolitanism is its middle name.
Jupiter cannot be petty, cannot ponder the daily minutia. It thinks big and dreams even bigger. Its optimism is contagious, sometimes reckless.
Jupiter is called the Great Benefic, the bringer of luck in astrology. Wherever Jupiter goes it brings luck and abundance with it. This association comes from Jupiter's size - it is the largest planet in our system, almost as large as the Sun. It expands whatever it touches.
In astrology, Jupiter merges everything into a single mélange. It distinguishes between nothing. All has its rightful place under the Sun. Live and let live. There is plenty to go and enough for everyone.
If Venus is about romantic love, Jupiter takes that love a step further and turns it into love for the human kind. Unconditional love in the case of Pisces, unconditional acceptance in the case of Sagittarius.
Everyone is welcome to the Sag-hosted party. Newcomers are greeted with openness. Foreignness is given freedom to express itself without fear of judgement or rejection. When that same concept is taken into the realm of emotions, the result is the unconditional Piscean love and understanding extended without expectation to all living creatures. People are not separated by any criteria, instead they are seen as one big party with a place for everyone. One more person, one more culture? Sure, throw it in the mix, the more the merrier.
Welcome to the global village. Everyone is invited.