Why would an ex block me? I haven't contacted him or made any advances but he randomly blocked me on instagram and facebook?

This could be due to him trying to let go. If he sees that you are active on social media, he may have the temptation to message you as he may still have feelings for you and to stop himself from doing this, he has blocked you. This does not mean that he doesn’t like you moreso the opposite, he still likes you so much that he has to remove the temptation of messaging you by blocking you. He may unblock you in a few weeks or months to have a look but I wouldn’t like to say how long.

An ex my block you because they find it hard to talk to someone that they once loved or still do love. You may have not done anything but they might feel if they got back in the relationship because they continued speaking to you it wouldn't be good for them. I say give that person time to grieve and get their thoughts in order. They will most likely unblock you, some people just grieve differently. In the mean time, focus on yourself and building yourself back up before trying to reach out to them. I say after the first month try to contact them but spend that month healing from the loss you just faced.

Sometimes, people find closure in doing exactly that. Blocking the person they don't feel alright about anymore, so they wouldn't be there for them to see. It happens that if you don't see something about or posted by someone, you won't think about them. He might feel like he can't handle memories of you or things that'll remind him of you two. It has happened to me before, so this is where this message comes from. Feel free to message me if you feel I might be able to help with your problem. I'm happy if I helped you with this post.