This is a true devotion to Xanga.

keeping the flames alive....
I will continue using the metaphor of fire because as humans we 'burn' energy even in our routine existence;
Fire needs 3 essential ingredients, these are observed in the 'fire triangle' - heat, oxygen & fuel cause the chain reaction that starts fire.
Heat could be said to be passion and one would say this is in the form of a mutual interest.
Oxygen is the aspiration which has to be matched evenly such as in the lyrics 'Breathe with me'.
Fuel is that essential other - something consumed, something that sets alight and then maintains the heat and aspiration.

In essence the answer is fuel as both heat and oxygen are already present.

If there are still problems, consider that fuel efficiency and expectation of a clean burn may require the periodic servicing of the grate.

If the couple are doing things together and somehow go off it - a mode of transport, things they buy, rote quips/reactions/manner

Indeed it does.... and it is most certainly God's best.... sometimes tho' life is not as black nor white as we would like it, and nor is following Christ.