Why is it that natural things like farts, poop, halitosis etc., from which we were always surrounded smell "bad"; whereas artificial stuff like parfume 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"?

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"?