It's rude to refuse things someone offers you.
Dave is at a work meeting writing notes when his pen runs out, which his coworker Tony notices and offers him his pen.
Dave wants to respond to Tony's offer.
Dave tells Tony thanks and accepts Tony's nice gesture of his pen.
Dave tells Tony that he would never touch a pen that's been in contact with another person's skin, it's gross.
Tony is happy that Dave has accepted the pen and was gracious enough to thank him for it.
Tony is insulted that Dave is insinuating he's dirty and that he's being so ungracious about his offer.