The continuous and non-continuous forms of certain English verbs have different meanings.

Compare:

When feel means ‘experience the condition of one’s body or mind’, it can be used in both continuous and non-continuous forms with little difference of meaning.

I feel fine. = I am feeling fine.

How do you feel? = How are you feeling?

When feel means ‘have an opinion’, it cannot be used in the continuous form.

I feel she is making a mistake. (NOT I am feeling she is making a mistake.)

When think means ‘have an opinion’, it cannot be used in the continuous form. Think can be used in the continuous form with other meanings.

What are you thinking about?

What do you think of my hair color? (= What is your opinion about my hair color?) (NOT What are you thinking of my hair color?)

When see means ‘understand’, it cannot be used in the continuous form. See can be used in the continuous form with other meanings.

I’m seeing Peter tomorrow. OR I will see Peter tomorrow.

I see what you mean. (= I understand what you mean.)

(NOT I am seeing what you mean.)

Smell can be used in the continuous form, when you are referring to the act of smelling with your nose.

Why are you smelling the fish?

In other cases, it is not used in the continuous form.

Does the fish smell bad?

When weigh means ‘have weight’, it is not used in the continuous form.

He weighs 65 kilos. (NOT He is weighing 65 kilos.)