The Question and Answer section for The Perks of Being a Wallflower is a great resource to ask questions, find answers, and discuss the novel.
I think he needed to show his feelings for Sam. Charlie is coming of age. He still has the whole sex and relationships thing to figure out. He did what he felt at the time.
What chapter are you referring to?
Charlie is an introvert. He does not get out and meet people easily. A Wallflower is a person who has no one to dance with or who feels shy, awkward, or excluded at a party.