I can think of a couple:
If you used the verb as an adjective, for example: I will ride my bike to swimming classes. "Swimming" is an adjective used to describe "classes".
If you're using the term "opposed to" or another verb that takes the preposition "to" (also called phrasal verbs), for example: I am not opposed to running. In that case, "running" is a gerund - a verb used as a noun - and it is the object of the verb "opposed to". Another example: I have become accustomed to drinking tea. Your example above is another - the idiom "look forward to" is a verb that takes the preposition "to". Because the verb takes an object, the verb becomes the object, and you use the "ing" form.
Hope that helps.