Particular English verbs require particular kinds of grammatical structure for their objects.
If the person who is to take the action is expressed, Recommend takes either a that clause or an infinitive:
I recommend that you read this carefully.
I recommend you to read this carefully.
But if the person is not expressed, it takes a noun phrase with a gerund (-ing) form:
I recommend reading this carefully.
There is no point in looking for a reason for this: it just happens to be a fact about the word recommend in current English. (Require and advise have the same pattern; suggest is similar, but won't take the to form; want takes only the to form).