When you become an adult there are a lot of changes in life that you will face. You will have to fend for yourself, make your own grocery list, do your own laundry and actually fold it (well maybe). You won't realize you are becoming an adult until it all of a sudden hits you.
Once you leave high school, done with college or just living on your own you will soon realize you were only friends with people because you saw them 5 days a week. When you become an adult your circle will get smaller, you may still have those you call friends but your trusting friend circle will become smaller and that is okay. You may find a significant other that you find you want to spend a majority of your time with. When you become an adult your S/O may become your husband or wife so it's normal to want to be your better half rather than your friends. Just because you want to spend time with your S/O does not mean that you don't love your friends because they will also find their better half. My personal group of friends grew smaller once I moved out on my own and my friends found their husband, fiance, not to mention themselves. I found my partner in crime and we went from being 3 friends to doubling to 6 friends and going out on dates together.
Our friendship now that we have become adults is so much better than our friendship in high school ever was. We talk about our lives, our career paths, our relationships and it really amazes me that we have grown up so much. However we still have our actual adult conversations about alcohol and house parties although our parties are a lot tamer now that we are adulting with our lives.
I realized I was an adult when I started spending more time in the Houseware aisle at Walmart than the Electronic aisle. My friends and I started talking about house decorating, and grocery shopping at Aldi's. I fell in love with Hobby Lobby and I realized that I could easily spend an entire paycheck at Hobby Lobby. My Pinterest board is now couponing boards, budgeting boards, cheap recipes, and boards about home rather than fantasy wedding boards, and clothing boards. I realized I became an adult when I refuse to buy the 68 cent toilet paper (it's not worth it). I realized I am an adult when one of my dream stores to visit is IKEA.
Being an adult isn't all that bad, yeah it's not nearly as great as being a child but like Monica said to Rachel in F.R.I.E.N.D.S "Welcome to the real world. It sucks. You're gonna love it."