Why can we use present simple for "I feel fine"?
Solution 1:
Both I feel fine and I'm feeling fine are grammatical (although this nGram shows the former is a lot more common than the latter).
Swan in Practical English Usage (p455) in 'Present tenses: Advanced points - section 7: I feel / I'm feeling' says:
Verbs that refer to physical feelings (e.g. feel, hurt, ache) can often be used in simple or progressive tenses without much difference of meaning.
- How do you feel? OR How are you feeling?
- My head aches. OR > My head is aching.