People who stare may be climbing imaginary stairs to some fantasy about "Young people these days!" or other such bullshit. They may be daydreaming. They may be old and lonely. They may be attracted to you. It's impossible to say without asking them, and, in truth, they may not have a solid handle on why they are staring. What we can say for sure is that if they are a stranger then 100% it has nothing to do with you or whomever they are staring at. Their reactions and responses are a thing they own in entirety.