A guy and a girl can be just friends, but at one point or another, they will fall for each other...Maybe temporarily, maybe at the wrong time, maybe too late, or maybe forever

This quote is saying that two people of the opposite sex can be friends. However, at some point during their friendship, they may develop feelings for each other. It is possible for those feelings to be fleeting, or for them to develop but for it to be the wrong time in their lives for a relationship. Or, it is also possible for them to fall in love and stay in love with each other forever.