Ah, that “Jeff smell”. This has remarkably little to do with showering routine and cleaning products, and almost everything to do with your personal sweat. In addition to water and electrolytes, sweat also contains small amounts of waste products, like urea, and smellier things like mercaptans easily pass through the skin. So diet plays a role, natural bacteria in the gut and on the skin play a role, organ health plays a role, density of apocrine glands, percentage of sugar in sweat (which, in turn, allows for increased growth of yeasts which produce their own smell), degree of keratin production (mostly because this can plug pores), natural hormones, and natural pheromones, among other things. Obviously, frequency of bathing will diminish the scents on people, and perfuming plays some minor role, but if you recognize your friend’s scent on their pillow, for example, it’s mostly due to what I just wrote.
In addition to this, there is the influence of breath, which people don’t normally think about. There are a lot of reactive species that get exhaled that have a scent, many of which will linger in air, including ketones, alcohols, and volatile organic compounds. This isn’t even including situations of poor oral hygiene or stomach upset, the latter of which can smell like HCl or sulfur, depending on the nature of the problem.
Top comment (by indianola) responding to the /r/ELI5 thread, “How come all my friends’ houses and clothes have a distinct, separate smell?”