Can Give it, but Can't Take it

At our store, if you return something or exchange something, you have to sign a piece of paper at the end of the transaction. So, I always have lots of pens handy. However, sometimes someone insists on using her own pen. I have no problem with someone who wants to use her own pen. But why do I feel like it has to do with her not wanting my germs after I've just taken back all of her items with hair and who knows what on the returns. So, she can use her own pen, and not become immune to my germs, but I will become immune to hers!