Throughout adult years people you meet, bond with, associate with and even take trips with.....come in and out of your immediate life. It doesn't mean you cast them off, they get busy, meet new friends, new interests and so does their mate, if they have one.
I have found that even if you haven't seen someone in several years, when you reconnect, that friendship is still there. The respect for each other is still there, and so you do a few things together again. Often, I remind myself that when you make a friend, unless they deliberately offend you in some way, they are making a pleasant life for themselves, and just because I am not involved, doesn't mean we are any less friends.
I would hate to think that because I don't call of invite them to join me every week, or month, they are less important to me. A good balance of time, interests, and interactions is very important to each one of us.
I have ended friendships over the years, simply because when it weighs on you, and you no longer enjoy their company, it's time to quietly break away. I haven't regretted my moves.