Flash-Bang.

Life, relationships, and sex are less about chemistry than they are about gravity, astronomy, and the physics thereof.

Two bodies attracted to each other, less force on each other the further away they are. Move them closer and the attraction is stronger. They circle and orbit, spiralling inward until they collide. 'Bang, energy.'

Bodies in motion will stay in motion unless acted upon by an outside force? Two people are in this great relationship. They'll stay in that great relationship until change takes place. Basic relationship sense. The change could be 'growing up' or 'growing apart' or that ever-pesky 'infidelity'. Whatever the reason, the two bodies have changed course.

The hotter a star is, the more energy it expends. The hotter is burns, the faster it burns out. The cooler it is, the slower it burns out. It may not burn out for several billion years due to the fact that it expends almost no energy in its burning.
The same can be said for a relationship. A very intense relationship will burn out very quickly or peter down to a simmer because all of the energy is expended very quickly in a short amount of time.



There's so much else that I can say on this subject. I'm neither educated enough nor unbusy enough to do so, though.