I'm not sure if you mean this in a general way, or if you mean that some people get more likes/comments than others. If it's the latter, then the reason could be that the content posted by these people is being seen more.
Some months ago, Facebook once again changed the display of the newsfeed. It changed in who you see posts from, as well as what posts you see of theirs. If you are subscribed to a page, you have probably learned that you need to choose to receive all updates/posts due to this update. Well, it's the same for your friends.
Facebook decided to start displaying posts from select friends, and only showing some posts. This is based upon how much you interact with them on FB, and how much you have in common. I'm not sure exactly how it's calculated, but these factor in. Just as you have to select to see all posts from a page, you need to visit a friend's page and select to see all posts by them. You can also opt to add them to your "Close Friends" group, and receive notifications every time they add a new post/picture.