The same reason you flinch and feel pain when you hurt yourself.
It's a reaction to your taste buds to something sharp and extreme in flavor such as a sour.
The oranges this season taste sour.
The term was coined in reference to the face someone made when they did infact taste something that had a sour taste i.e. a gooseberry. Thereafter people who made a face when they were upset or just pouting were said to have had a "sour face".
Yes, the sentence is in the present tense.
If you bite into a lemon, you make a face on account of the sour taste. If you buy something that doesn'twork out for you, you may also make a sour face.
taste
It will still taste bad even if it is served with sour cream. it would have to be rotten as sour cream is nasty at its best
Sour taste is a result of a chemical property. Certain chemicals - acids in particular - produce a sour taste when they come in contact with the appropriate taste buds. The chemical interaction of those chemicals with the taste buds yeilds a stimulus of those taste buds that the brain interprets as "sour".
A sour face is usually pursed lips, squinted eyes, and sucked in cheeks. It doesn't necessarily mean one ate something sour, but it looks as if they did.
Acids have a sour taste.
Yes, raspberries can be sour in taste.
Sour *is* an adjective. It has an adverb form, which is sourly.
The future progressive tense of "sour" is "will be souring."