I hear what you are saying Scarlett and can relate to it. I think it's the kind of 'Oh, they like me, that's great' high when you find a new friend, they don't rely on you too much to begin with and everything you talk about is exciting and fun. Then over time you get to really see the person your new friend is and they might start to rely on you for help and support, and you to them.
The first stage is a fun friend, the second stage is a True friend. The plateau comes when you are comfortable with your friend and no longer need to impress them and always try to be enthusiastic about everything.
You have to take the high and the lows and make a commitment to be there for your friend, that's what friends do. I lost contact with a lot of friends about 15 years ago when I needed help and support and they weren't there like I'd been for them. The people that stood by me and still do are my true friends, the people whom didn't want to hang about were my 'fun' friends as they didn't want to commit.
Apparently research shows that we go through a set of 'fun' friends every 7-10 years as we move through life and keep a small handful of friends as life-longer's Strange, but true.