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 Opposition Neptune
A Sun opposition Neptune aspect can raise issues of trust. Sexual urges can be very compelling but your dream of romantic relationship can become so fanciful and inspiring that it may cause you both to lose your sense of proportion and reality. If their Sun is in opposition to the Neptune person, it may be easy for you to accept as fact what you desire rather than what is actually true based upon intuitive feelings about how things could be rather than the facts as they now are. This tends to be a difficult combination. The Neptune person has a subtle psychic effect on the Sun person but may create much confusion and misunderstanding.