There are many reasons why someone might unfriend us.
For example, we might unfriend a person because we’re bothered by what he/she posts. Could that be the case here? Are you posting too much personal information? Do you rant about sensitive topics? Do you regularly share your game scores? People don’t all have the same interests.
You might also reconsider your bond with the other person: are you really all that close? Is this someone you chat with regularly? Sometimes people just decide to pare down their contact lists and they simply remove people they don’t see very often.
If you aren’t close to the person, don’t feel personally targeted – just move on. If it’s a good friend and you’re worried, however, you can talk to him/her to find out what’s going on.