Why is my ex acting this way?

My ex and I got in an argument on Monday and broke up in the heat of the moment (we had broken up before but always talked after and got back together). However. This time, he messaged me after and I stopped replying after a while. Then on Thursday, he replied to a selfie I posted on my story and told me he had a fever. I left him on read. But why is he acting so casual after we ended badly? Why even message me?
Why is my ex acting this way?
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