I think reading is very interesting. Why are there two verbs in this sentence?

Actually this sentence is an object clause.

The main clause is "I think" and the subordinate clause is "Reading is very interesting".

The two verbs "thinking" and "reading" in the sentence have different functions.

The first verb "think" is the predicate of the object clause, and the second verb "reading books" is the gerund phrase of the subject of the clause.

But this object clause omits the conjunction "that", that is:

I think reading is very interesting.

Among them, the object clause is a sentence with systematic structure, and the subject is "reading", that is, the subject of gerund phrases.

And "interesting" is an adjective, meaning "interesting".