interested in certain topics? Most likely it's a combination of both but I believe human beings are generally more interest driven.
When Collections were introduced to Google+ you could read stuff like: "Ah, now they're trying to copy pinterest because they failed to beat Facebook." Especially tech media goes for easy headlines and explanations which were made to serve first they're own purposes.
In the meantime many have realized that Collections are a powerful tool and a lot have been created. The old "follow you - follow me" game seems to be over. Circle sharing is already over (thank you Google!!) and this is a clear sign that Google+ is moving towards being an interest based system instead of a follower based one.
Facebook connects people and Google+ becomes the network where primarily common interests are shared and of course also people get connected but this happens as a "side effect".
I personally really like Collections and have created so far
9 (see above) in my profile and
5 (see below) in my page
and I'm sure there will be more.