Explanation

In the above example, the preposition to is part of the prepositional verb look forward to (something or doing something). Since to is a preposition, it must be followed by a noun or a gerund (i.e., the -ing form of a verb).

Common Prepositional Verbs

Prepositional verbs are quite common in English, and learning the correct verb / preposition combinations is a matter of memorization and practice.

Prepositional verbs consist of two parts – a verb and a preposition. A noun, pronoun, or gerund can follow the preposition. Below are some common combinations:

look forward to	get on / off
look at	go into
look after	deal / cope with
complain about	care for / about
depend on	think about

Examples (verb + preposition + gerund):

I am thinking about going to Spain next year.
She cares about doing good things for others.
It depends a lot on having good luck.
He always complains about going to work.
I look forward to meeting you soon.

Examples (verb + preposition + noun / pronoun)

It depends on the cost.
She always thinks about others.
Mark’s father is ill. He told me that he looks after him every day.