The Sun is the source of all energies. These energies stimulate the activities of the houses occupied by the transiting Sun and reinforce or weaken the planetary effects, depending on the Sun's aspect to the natal planet. When the Sun transits an inner planet, it may trigger a dormant aspect between that inner planet and a slower moving outer planet. If a planet is being transited by another planet when it is being transited by the Sun, the effect of the transit is strengthened.
Sun Square Jupiter
A Sun square Jupiter aspect causes differences which are a challenge to your ideas about romantic relationships. Both partners will feel right and find it hard to understand why it is so difficult for the other partner to understand their basic philosophy of what a relationship should be like. When the Sun is square the Jupiter person their energies will seem to blow minor issues out of proportion. You tend to encourage each others tendencies to self-indulgence and impracticality. You are not disciplined or practical and ignore responsibility.