why we breathe oxygen and not something like carbon dioxide or nitrogen?
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One of the main drivers in chemistry is the way the electrons group up in an atom. They don’t just go random, they fill in zones. For various reasons, the zones form stable groups (have “magic” numbers, in a way, 2, 10 and 18 are the number most small elements most want, if they can get it, and getting to the closest magic number of electrons is pretty much what makes reactions happen). having a particular number of electrons (“magic” numbers) is more stable than having one or two more, or less, than the “magic” number.
So, what happens when elements react and form compounds, is some elements will become more stable (get to a “magic” number) by grabbing an electron or more if they can find one, whereas others have only a weak hold on the one or two (or more) electrons that put them beyond the ideal stable number, so the elements with a big need GRAB an electron or more from elements that only have a weak hold, and both elements become more stable in the process. For example, oxygen has 8 to begin with, so it kind of really would prefer to have 10 if it could. Carbon has 6, so it could go either way, taking 4 to get to 10, or losing 4 to get to 2.
In the big mix of the world of all elements, there is a fight for electrons. Strongest ones win. Oxygen is really strong at attracting electrons (only fluorine, the next element over in the periodic table, with one more electron that oxygen, pulls harder, but it is a fairly uncommon element).
So, oxygen, because it is very common and wants to react with almost anything it can, is the basis for a lot of chemistry (we even name a major type of chemical reaction, “oxidation” on what happens when oxygen reacts with other elements by stealing electrons).
A lot of other things come into play, of course, but the fight for electrons is a big reason for a lot of chemical reactions. You need to know, though, that once an element has stolen (or lost) electrons and moved into its most stable condition, it won’t fight anymore, so no longer will react.
So we breath in oxygen as O2 (has not stolen electrons from anywhere yet) and it is used to steal electrons from, well, mostly carbon. Carbon’s electron number is plop in the middle of the stable range so will benefit by either adding 4 electrons or losing 4 electrons, so can go either way. It does not have a good strength for stealing electrons and doesn’t have a good strength for keeping electrons, so which way it will go depends on what else is around that is fighting for electrons. Carbon is an average fighter, not a weak fighter like sodium, say, but not a brute like oxygen either.
Life chemistry uses this to its advantage, making sugars using sunlight (so carbon is acting more like an electron thief in sugar, with the help of solar energy) but getting energy back by “burning” carbon with oxygen. When oxygen combines with carbon, the end result is carbon dioxide. Carbon dioxide is pretty stable and won’t react more without putting in energy (going back to C and O2 is the wrong way on the energy hill so something has to push it back up, and plants do the pushing using energy from the sun).
Nitrogen is sort of in between carbon and oxygen in how strong it is at stealing electrons (nitrogen “wants” 3 electrons but oxygen “wants” only 2) , so can be a source of energy through oxidation of something else, but it is less energy that you get by doing the same thing with oxygen. There are also issues with the shapes of structures made using nitrogen that make it less useful (compare ammonia NH3, a triangle in shape with nitrogen in the middle so not really polarized, to water H2O which has oxygen in the middle and is polarized by having extra electrons on one side and H on the other). Nitrogen is still a very important element in organic chemistry (chemistry of living things) but it is not a major energy source.
So, we breath in oxygen as O2 (wants more electrons) and breath out oxygen as CO2 and water (H2O) both of which are as stable as they will get – all the elements involved have the best number of electrons for their make-up; C gave 4 electrons to get down to 2, which each oxygen took 2 to get to 10, making CO2, or with water/H2O, each H gave one electron (the only one it had, and went to 0) and the O took 2 going to 10. Everybody is as happy as they can be, until plants come along and zap some solar energy back into the mix and undo all the work.
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