Why is it that natural things like farts, poop, halitosis etc., from
which we are always surrounded smell "bad"; whereas manufactured
products, such as perfume or glue smell "good" to most of us?
In my understanding, things smell "bad", because they are not good for
us. But how are farts not good? And shouldn't artificial things like
perfume or glue, which are relatively new to us, smell "bad"?
Kind of a strange question but here is a quick and rough answer... There are at least three things to think of here.
Firstly, yes often natural things that are good for you have evolved to smell attractive, but not all natural things are good for you. Therefore a fart should not smell good just because it is natural. And it is likely that breathing farts non stop is not good for you.
Secondly, perfumes etc. have been chemically manipulated or designed to smell nice by human manufacturers - the preference for such smells existed before the specific perfume, the manufacturer simply designed a fragrance that was attractive.
And Finally... Not everything exists because of selection. Farts probably smell as a by-product of process of digestion rather than selection for avoiding them. Gas is produced during the digestion processes, the amount and odor (determined by the constituents of the gas) of gas will likely depend the efficiency of digestion and what is being digested. The dominant selection force affecting the odor is then probably related to digestive efficacy and selection against odor would only be (relatively) very weak.
It's better to digest food and fart a bit than it is to not digest but smell nice, digestion is quite important!