It’s all about electrochemical potential. Adding or removing electrons from the outer shell of an atom involves a fair amount of energy, either being released or stored. Depending on the atom, they either want their shell filled or emptied. Noble gases have the same number of protons as a filled shell, so they are very stable. Why electron shells exist is a whole other matter.
Water is kind of like ash. Technically full combustion of any hydrocarbon outputs CO2 and Water. Since water isn’t a greenhouse gas it’s not mentioned when discussing combustion usually.
This is one of the reasons why methane leaks are so impactful - not only is methane a terrible green house gas, when it decays in the upper atmosphere, it decays into water vapour and CO2)
Water is kind of like ash. Technically full combustion of any hydrocarbon outputs CO2 and Water. Since water isn’t a greenhouse gas it’s not mentioned when discussing combustion usually.