1) I consider them a friend, meaning we hang out or we would if we lived in the same area, or at least we travel in the same social circles or
2) I consider them interesting or cool enough to be worth following even lacking that, due to common interests/topics
Sometimes these things change, and sometimes following people becomes painful or irritating for some reason, or there was some insult, or something like that. Sometimes this is also because I was always hoping for more, and it's an easy way to stop hoping that if I stop hearing words directed to circles of friends I never really was a part of anyhow (or which I exited somehow).
As usual, follow me if you like, don't if you prefer not to, chances are we'll drift apart if we lose all means of communication, but that's probably okay anyhow. Few social ties last forever, nor are they meant to. If our lives eventually drift back together, (probably) great; we can grab tea and talk philosophy and politics and current events and activism and books and music and food and take walks and climb rocks and go camping and all the other things I like doing with friends (or at least liked, the last time I had a functional circle of friends). If not, no real worries.