Dear Egg (another of my favourite food
),
I don't really understand what she's talking about leh 
The so-called "sparks" actually make up chemistry between 2 people.
Chemistry in a love context is kinda like its scientific
counterpart. Like water = chemical bonds between hydrogen &
oxygen, chemistry in relationships is about a guy & a girl
coming together & forming a hopefully lasting & unbreakable
bond.
If something extreme happens (like in science, extreme heat is
applied, etc), then the bonds may break, & hydrogen &
oxygen hafta be by their lonesomes again until hydrogen finds
another oxygen to bond with.
Chemistry would refer to the compatibility (water can't be formed
with hydrogen & sulphur) between the 2 parties, & the
circumstances (like extreme heat applied) under which the
relationship is formed.
FeowFeow