When you notice people you cared become distant from you, what do you do? And what are your thoughts?

Distance yourself also, and give as good as you get is my advice to you. Of course follow people up with a call or email etc. but leave it up to them to respond and don't bother with them if they're too wrapped up in their own worlds to make the effort. Some people do change their associations over time, but true friends will always be there for you.