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 Mercury
A Sun opposition Mercury aspect can be very difficult for marriage unless you can maintain a certain level of emotional detachment from the subjects you discuss. You will definitely see two sides to every story. Your intellectual relationship is what stimulates you, sharing thoughts, studying together, sparking each others curiosity and interest is what your relationship is about. When the Sun person is in opposition to the Mercury person, you will be made aware of ideas you may never have thought about on your own and could find yourself defending your viewpoints in most of your conversations.