I think by being nice, we find out if there is more for us in meeting someone than that. As long as there is no malice, I think we need to (for our own self-worth, and safety) presume the other person has reasons that likely have nothing to do with us. (don't know why, but the song Lydia and Dave by John Prine popped into my head, it is a serious/thoughtful song about two people far apart who are suffering the same loneliness----written a long time ago, I can recall an occasion when there was an audience member who did not "get" the sincerity of the song (or was uncomfortable) and laughed---the audience quickly shushed him down.)
In order to make friends we do need to be vulnerable, able to laugh, cry, enjoy, and grieve. People come and go---and you really never know when someone feels about you as you may feel about someone you want to know better.
I have a friend/acquaintance of many years, and mutual respect; who once said to me (about my then husband and myself) "R. and I tried so hard with you guys, to be friends." I felt mortified because I didn't "get" it---I somehow thought I had tried...but over the years, I can see what she meant. They invited us to dinner, to parties, we did go (mostly) but we were not comfortable enough in ourselves to invite them back --- to return the favors---
...and other things. But, now is now, and I am glad for the friendly acquaintance status, wishing, at times, I had known better back then...I really didn't think I was someone anyone like these great folks would want hanging around...
I have learned to take what comes, let go as I must.
Some of this, I am sure, is related to the transitional nature of our culture/society/not growing up in small towns for generations/or even city neighborhoods of similar duration where proximity makes for connections.