How can I not get so upset when people don't acknowledge me?

Hello everyone,

I get so bothered when a stranger doesn't really acknowledge me. One time for example, I was with my brother at the grocery store. I went to the checkout line, said hi to the cashier, then she bags my groceries. Then my brother gives the cashier his alternate id for the grocery store, since he was a member, to make my groceries cheaper. Then once she gave me my change and my receipt I tell her "Thank you". She said "You're welcome" in a quiet tone, but then she looks at my brother, smiles, and loudly tells him "Thank you have a nice day dear!", as if he was the one who purchased the groceries. She wasn't at all cheery with me. Another time I was with him purchasing some school supplies, and I went up to the cashier to purchase my items. The cashier was on the phone, but he did make eye contact with me. However, he didn't really give me a friendly look. After I purchased my items, and he gave me my change, he looks at my brother, smiles, and tells him "Thank you" as if he was the one who purchased the supplies. I was out to eat with my cousin once, and when the waitress came up to our table she tells my cousin "Hi, how are you today?!" Completely ignoring me. I'll get irritated when I'm out with another person, and we walk into a store, and the employee only acknowledges the person I'm with and not me. I get so frustrated when this happens to the point where I think about it all day. It'll make me happy when they acknowledge me and be friendly with me. I hate being ignored like I don't exist. When people do this, I think there's something wrong with me. So what do you guys think? Why does this happen to me? How can I change this and how do I not let it bother me so much?