How do you pick your posts??

So I'm wondering, how do you pick the posts you comment on? Do you just see what pops up on Stories/Questions (that's what I do) or do you go to groups or certain users profiles?? Here's my confusion...it seems that a great deal of the same people respond to my posts and I see their posts. However, there are some people who respond to me regularly but I NEVER see their posts pop up, I will go actively seek them out but why aren't their posts showing up for me? (I can see them if I go to their profile) We're friends, we interact in the public forum, we seem to be in at least some of the same groups...what gives??