Jupiter is supposed to be benefic. It's "good luck". You probably feel great and blessed during Jupiter transits. So when it is transiting in the 7th house of relationships, marriage and business partnerships, essentially your significant one-to-one relationships are supposed to go great during this period of time! Is it then possible for a breakup to "happen" during this time?
Jupiter is concerned with expansion (of the self or perhaps more accurately, the boundaries of self), improvement, philosophy including religion, life's ambitions and plans, etc. If a relationship isn't expansive or growth-oriented for a person, perhaps there is an opportunity for the person to discontinue the relationship if she/he chooses to. I don't think there's "something" to compel an individual to breakup. In fact, I have read somewhere where an astrologer describes Jupiter as a lazy planet. It could be the case where the person is aware that a relationship is no longer conducive towards her/his growth but is reluctant to take any action to improve it.
So, yes, a breakup is a possibility when Jupiter transits the 7th. But really, above all, it's a time of opportunities to be expansive and optimistic in the sphere of relationships.
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