Not being invited to a party is a form of rejection that is quite frankly telling. It says I don't enjoy your company, you're not important to me, and I don't care if you know that. If you're not going to invite someone to a big party you're throwing, expect them to be upset about it.
My (32f) daughter (16f) grew up extremely close to my sister (23f) because of how small the age gap between them is, and my sister got married in September. However, she decided to have a child-free wedding, and my daughter was not invited.
My daughter was extremely upset about this as my sister always talked about how important my daughter being at her wedding was going to be, and her child-free wedding came out of the left field.