Excess oxygen is actually harmful to humans, but all the climate warnings are about losing oxygen, not nitrogen edit: but when we look for habitable planets, our focus is ‘oxygen rich atmosphere’, not ‘nitrogen rich’, and in medical settings, we’re always concerned about low oxygen, not nitrogen.
Deep sea divers also use a nitrogen mix (nitrox) to stay alive and help prevent the bends, so nitrogen seems pretty important.
It seems weird that our main focus is oxygen when our main air intake is nitrogen. What am I missing?
edit: my climate example was poor and I think misleading. Added a better example instead.
N2 is (mostly) inert when it comes to respiration. What your body needs is oxygen and low concentrations of anything that might also be metabolically active. For scuba diving, N2 is used to dilute the oxygen and is used specifically because of how non-reactive it is. At high concentrations though, it can result in nitrogen narcosis - helium is sometimes used as the diluent gas instead to mitigate this.
As far as habitability is concerned, atmospheric nitrogen is essential for life on Earth at least, as it’s a major part of the nitrogen cycle (specifically, nitrogen fixation). Without it, we wouldn’t have nitrogen-containing organic compounds like amino acids (and, therefore, proteins), at least not nearly in the same quantities that we currently do. This doesn’t mean it’s essential for life outside earth, but it is for life as we know it, so its presence should increase our credence (if only a little) for whether a given planet is habitable or not. However, when looking for signs of life, it’s better to look for atmospheric signatures that are heavily influenced by life, rather than just those that facilitate it. The oxygen in Earth’s atmosphere was largely produced by life, and so its presence in the atmospheres of other planets would be a good (though not definitive) indication of habitability.