Its possible they could be shy, or maybe they are going through some weird situation on their own.
They could have friend swings (mood swing version of wanting or not wanting friends lol) like me. I'm really ... awkward ... when it comes to talking to people at times. I have trouble adding certain people or starting conversations with people I've seen around for a long while. I feel that after I've waited too long then it just..is too difficult for me to add them.
I've deleted people off of my friends list before but not because I was trying to be mean. Its cause I've had weird..break down..sort of things that had nothing to do with them at all. Then I felt bad and added most of them back. I've recently tried adding people which was a success at times then a fail at other times.
If that person is anything like me of course they will puzzle you
. But its more than likely nothing against you on purpose.