To keep some relationships strong over time, there will be a movement from familiarity to formality. For example, when your children are living under your roof, there is great familiarity, but when they start their own lives as young adults, there needs to be greater formality. This is not a stiff, unfriendly formality, but instead a formaility that includes respect and etiquette as your adult children build their own lives. This same movement from familiarity to formality happens with siblings and childhood friends as you get older and people's lives change and become entwined with so many others.
The used-to-be familiary didn't include as much planning and preparation, but instead was more of a get-up-and-go time when you simply rallied the children, hopped into the car, and went to the beach or the mountains. Now, with adult children, that requires planning--you have to have that talk about when they can make it and how they want the event to go. The same is true with your siblings--there was a time when you and your siblings were part of a tight family group with similar traditions, but as you grew older people partnered-up, developed their personal interests and routines, and moved in multiple directions. So now a get-together with siblings involves more planning, preparation, and respect--it's not the way it once was. And, the same is true with childhood friends--you once lived near each other and had similar schedules and lifestyles, but with time lots of variability entered the equation which did create a movement from familiarity to greater formality.
Sometimes this movement from familiarity to formality strikes you with surprise--you're not quite ready for it, and then it becomes a reality. In the best of circumstances, you're aware of this gradual change, and embrace it to keep your wonderful relationships strong as times and places change. Onward.