The infinitive is used for different reasons.
It can be used :

– as an object (eg. I like to sing.) It answers the question : What do you like doing ?

– to complement a verb (eg. My passion is to sing —> to sing complements “is”.)

– to qualify a verb (eg. I live to sing.) It answers the question : Why do you live ?

– to qualify an adjective (eg. It is great to sing.) It answers the question : What is great ?

– to qualify a noun (eg. I haven’t got enough time to sing.) It answers the question : What havent you got enough time for ?