Jupiter is the planet of expansion, opportunity, and fun. Jupiter doesn't want to take too much too seriously, and wants to get out and explore, experience life, and open up to the world. Jupiter wants to goof off and be playful.
In love, this means Jupiter is the planet that helps us be romantic, find new people to date, and have fun flings. With Jupiter, love is fun and light and not too scary or serious. That's the job of other planets. Jupiter's job is to help us enjoy each other.
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Jupiter Transits & Love
Meeting someone and dating someone new and exciting can come with Jupiter aspects (conjunctions, sextiles, and trines) to the natal or progressed 5th house cusp or ruler (house of love), 7th house cusp or ruler (house of relationships and commitment), or natal Venus (planet of love and relationships).
When transit Jupiter is touring the natal 5th house of love, this is often a period where you can meet lots of potential love interests and be more open to love; when transit Jupiter is touring the 7th house of relationships, this is often a period where you can be lighter about commitment, more open to it, but desire some space within it; and when transit Jupiter is touring the 8th house of intimacy, this is often a period where you're more open to intimacy (and sex!). But note that Jupiter is the planet of expansion, so if you're having problems in love (5th house), relationships (7th house), or intimacy (8th house), Jupiter touring these houses can make them worse!
Hard Jupiter transits (squares and oppositions) can make it so you don't really want any strings attached since Jupiter loves freedom, or can make it so you're lazy about any new connections and don't pursue them, or lazy about issues in existing relationships and don't work on them. Jupiter needs a push under the hard transits, otherwise things can get stale and nothing moves.
Jupiter & Synastry
When you have a Jupiter connection with someone in synastry, this is one of the best indicators of compatibility. We often get along very well with the people who are of the same sign as our natal Jupiter, and it can generate a genuine like between you. It's not usually unrealistic, rose-colored glasses kind of like (like Neptune); we usually see the flaws in one another, but don't mind them so much. We still like ya anyway!
Jupiter isn't really terrible in love astrology at all, BUT without other planets to ground the energy, it can cause new relationships to be fleeting flings. Saturn is usually needed to counter Jupiter since Saturn is super committed and grounding. Pluto can also be of help since Pluto is also a serious planet, and wants strong connections.
Having positive Jupiter transits combined with Saturn and/or Pluto can be one of the best indicators of a new relationship that will last or of advancing an existing relationship. But without Saturn or Pluto? Enjoy the ride - it probably won't last!
Suggested Reading: Articles in the love astrology series: Mars, Saturn, Uranus, Neptune, Pluto, the 12th house, Chiron, eclipses, retrogrades, the Lunar Nodes, Ceres, and the Asteroids of Your Name